Dark matter

Dark matter

Overview
In astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 and cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

, dark matter is matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 that neither emits nor scatters
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

 light or other electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

, and so cannot be directly detected via optical or radio astronomy. Dark matter is believed to constitute 83% of the matter in the universe.

Dark matter was postulated by Fritz Zwicky
Fritz Zwicky
Fritz Zwicky was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and observational astronomy.- Biography :Fritz Zwicky was born in Varna, Bulgaria to a Swiss father....

 in 1934 to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...

. Subsequently, other observations have indicated the presence of dark matter in the universe; these observations include the rotational speeds of galaxies, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster
Bullet cluster
The Bullet cluster consists of two colliding clusters of galaxies. Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of dark matter...

, and the temperature distribution of hot gas in galaxies and clusters of galaxies.

Dark matter's existence is inferred from gravitational effects on visible matter and gravitational lensing of background radiation, and was originally hypothesized to account for discrepancies between calculations of the mass of galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

, clusters of galaxies
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...

 and the entire universe made through dynamical
Dynamics (mechanics)
In the field of physics, the study of the causes of motion and changes in motion is dynamics. In other words the study of forces and why objects are in motion. Dynamics includes the study of the effect of torques on motion...

 and general relativistic
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

 means, and calculations based on the mass of the visible "luminous" matter these objects contain: star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s and the gas and dust of the interstellar
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

 and intergalactic medium.
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Encyclopedia
In astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 and cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

, dark matter is matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 that neither emits nor scatters
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

 light or other electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

, and so cannot be directly detected via optical or radio astronomy. Dark matter is believed to constitute 83% of the matter in the universe.

Dark matter was postulated by Fritz Zwicky
Fritz Zwicky
Fritz Zwicky was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and observational astronomy.- Biography :Fritz Zwicky was born in Varna, Bulgaria to a Swiss father....

 in 1934 to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...

. Subsequently, other observations have indicated the presence of dark matter in the universe; these observations include the rotational speeds of galaxies, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster
Bullet cluster
The Bullet cluster consists of two colliding clusters of galaxies. Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of dark matter...

, and the temperature distribution of hot gas in galaxies and clusters of galaxies.

Overview


Dark matter's existence is inferred from gravitational effects on visible matter and gravitational lensing of background radiation, and was originally hypothesized to account for discrepancies between calculations of the mass of galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

, clusters of galaxies
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...

 and the entire universe made through dynamical
Dynamics (mechanics)
In the field of physics, the study of the causes of motion and changes in motion is dynamics. In other words the study of forces and why objects are in motion. Dynamics includes the study of the effect of torques on motion...

 and general relativistic
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

 means, and calculations based on the mass of the visible "luminous" matter these objects contain: star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s and the gas and dust of the interstellar
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

 and intergalactic medium. The most widely accepted explanation for these phenomena is that dark matter exists and that it is most likely composed of heavy particles that interact only through the weak force
Weak interaction
Weak interaction , is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars...

 and gravity; however, alternate explanations have been proposed, and there is not yet sufficient experimental evidence to determine which is correct. Many experiments to detect proposed dark matter particles through non-gravitational means are underway.

According to observations of structures larger than solar systems, as well as Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 cosmology interpreted under the Friedmann equations
Friedmann equations
The Friedmann equations are a set of equations in physical cosmology that govern the expansion of space in homogeneous and isotropic models of the universe within the context of general relativity...

 and the FLRW metric, dark matter accounts for 23% of the mass-energy density
Mass-energy equivalence
In physics, mass–energy equivalence is the concept that the mass of a body is a measure of its energy content. In this concept, mass is a property of all energy, and energy is a property of all mass, and the two properties are connected by a constant...

 of the observable universe
Observable universe
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...

. In comparison, ordinary matter accounts for only 4.6% of the mass-energy density of the observable universe, with the remainder being attributable to dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...

. From these figures, dark matter constitutes 83%, (23/(23+4.6)), of the matter in the universe, whereas ordinary matter makes up only 17%.

Dark matter plays a central role in state-of-the-art modeling of structure formation
Structure formation
Structure formation refers to a fundamental problem in physical cosmology. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.7 Gyr ago...

 and galaxy evolution
Galaxy formation and evolution
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby...

, and has measurable effects on the anisotropies
Anisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...

 observed in the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation. The largest part of dark matter, which by definition does not interact with electromagnetic radiation, is not only "dark" but also by definition, utterly transparent.

As important as dark matter is believed to be in the cosmos, direct evidence of its existence and a concrete understanding of its nature have remained elusive. Though the theory of dark matter remains the most widely accepted theory to explain the anomalies in observed galactic rotation, some alternative theoretical approaches have been developed which broadly fall into the categories of modified gravitational laws, and quantum gravitational laws.

Baryonic and nonbaryonic dark matter


A small proportion of dark matter may be baryonic dark matter
Baryonic dark matter
In astronomy and cosmology, baryonic dark matter is dark matter composed of baryons, i.e. protons and neutrons and combinations of these, such as non-emitting ordinary atoms...

: astronomical bodies, such as massive compact halo object
Massive compact halo object
Massive astrophysical compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter, which emits little or no radiation and drifts through interstellar...

s, that are composed of ordinary matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 but which emit little or no electromagnetic radiation. The vast majority of dark matter in the universe is believed to be nonbaryonic
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...

, and thus not formed out of atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s. It is also believed that it does not interact with ordinary matter via electromagnetic forces; in particular, dark matter particles do not carry any electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

. The nonbaryonic dark matter includes neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...

s, and possibly hypothetical entities such as axion
Axion
The axion is a hypothetical elementary particle postulated by the Peccei-Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics...

s, or supersymmetric
Supersymmetry
In particle physics, supersymmetry is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartners...

 particles. Unlike baryonic dark matter
Baryonic dark matter
In astronomy and cosmology, baryonic dark matter is dark matter composed of baryons, i.e. protons and neutrons and combinations of these, such as non-emitting ordinary atoms...

, nonbaryonic dark matter does not contribute to the formation of the element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

s in the early universe ("Big Bang nucleosynthesis
Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of H-1 during the early phases of the universe...

") and so its presence is revealed only via its gravitational attraction. In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation
Annihilation
Annihilation is defined as "total destruction" or "complete obliteration" of an object; having its root in the Latin nihil . A literal translation is "to make into nothing"....

 interactions with themselves resulting in observable by-products such as photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s and neutrinos ("indirect detection").

Nonbaryonic dark matter is classified in terms of the mass of the particle(s) that is assumed to make it up, and/or the typical velocity dispersion of those particles (since more massive particles move more slowly). There are three prominent hypotheses on nonbaryonic dark matter, called Hot Dark Matter
Hot dark matter
Hot dark matter is a hypothetical form of dark matter which consists of particles that travel with ultrarelativistic velocities. The best candidate for the identity of hot dark matter is the neutrino. Neutrinos have very small masses, and do not take part in two of the four fundamental forces, the...

 (HDM), Warm Dark Matter
Warm dark matter
Warm dark matter is a hypothesized form of dark matter that has properties intermediate between those of hot dark matter and cold dark matter, causing structure formation to occur bottom-up from above their free-streaming scale, and top-down below their free streaming scale. The most common WDM...

 (WDM), and Cold Dark Matter
Cold dark matter
Cold dark matter is the improvement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation and whose constituent particles move slowly...

 (CDM); some combination of these is also possible. The most widely discussed models for nonbaryonic dark matter are based on the Cold Dark Matter hypothesis, and the corresponding particle is most commonly assumed to be a neutralino
Neutralino
In particle physics, the neutralino is a hypothetical particle predicted by supersymmetry. There are four neutralinos that are fermions and are electrically neutral, the lightest of which is typically stable...

. Hot dark matter might consist of (massive) neutrinos. Cold dark matter would lead to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario.

One possibility is that cold dark matter could consist of primordial black holes in the range of 1014 kg to 1023 kg. Being within the range of an asteroid's mass, they would be small enough to pass through objects like stars, with minimal impact on the star itself. These black holes may have formed shortly after the big bang when the energy density was great enough to form black holes directly from density variations, instead of from star collapse. In vast numbers they could account for the missing mass necessary to explain star motions in galaxies and gravitational lensing effects.

Observational evidence


The first person to provide evidence and infer the presence of dark matter was Swiss astrophysicist Fritz Zwicky
Fritz Zwicky
Fritz Zwicky was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and observational astronomy.- Biography :Fritz Zwicky was born in Varna, Bulgaria to a Swiss father....

, of the California Institute of Technology
California Institute of Technology
The California Institute of Technology is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering...

 in 1933. He applied the virial theorem to the Coma cluster of galaxies and obtained evidence of unseen mass. Zwicky estimated the cluster's total mass based on the motions of galaxies near its edge and compared that estimate to one based on the number of galaxies and total brightness of the cluster. He found that there was about 400 times more estimated mass than was visually observable. The gravity of the visible galaxies in the cluster would be far too small for such fast orbits, so something extra was required. This is known as the "missing mass problem". Based on these conclusions, Zwicky inferred that there must be some non-visible form of matter which would provide enough of the mass and gravity to hold the cluster together.

Much of the evidence for dark matter comes from the study of the motions of galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

. Many of these appear to be fairly uniform, so by the virial theorem the total kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

 should be half the total gravitational binding energy
Gravitational binding energy
The gravitational binding energy of an object consisting of loose material, held together by gravity alone, is the amount of energy required to pull all of the material apart, to infinity...

 of the galaxies. Experimentally, however, the total kinetic energy is found to be much greater: in particular, assuming the gravitational mass is due to only the visible matter of the galaxy, stars far from the center of galaxies have much higher velocities than predicted by the virial theorem. Galactic rotation curves, which illustrate the velocity of rotation versus the distance from the galactic center, cannot be explained by only the visible matter. Assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this. Galaxies show signs of being composed largely of a roughly spherically symmetric, centrally concentrated halo
Dark matter halo
A dark matter halo is a hypothetical component of a galaxy, which extends beyond the edge of the visible galaxy and dominates the total mass. Since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas...

 of dark matter with the visible matter concentrated in a disc at the center. Low surface brightness dwarf galaxies
Low surface brightness galaxy
A low surface brightness galaxy, or LSB galaxy, is a diffuse galaxy with a surface brightness that, when viewed from Earth, is at least one magnitude lower than the ambient night sky....

 are important sources of information for studying dark matter, as they have an uncommonly low ratio of visible matter to dark matter, and have few bright stars at the center which would otherwise impair observations of the rotation curve of outlying stars.

Gravitational lens
Gravitational lens
A gravitational lens refers to a distribution of matter between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer...

ing observations of galaxy cluster
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...

s allow direct estimates of the gravitational mass based on its effect on light from background galaxies, since large collections of matter (dark or otherwise) will gravitationally deflect light. In clusters such as Abell 1689
Abell 1689
Abell 1689 is a galaxy cluster in the constellation Virgo. It is one of the biggest and most massive galaxy clusters known and acts as a gravitational lens, distorting the images of galaxies that lie behind it...

, lensing observations confirm the presence of considerably more mass than is indicated by the clusters' light alone. In the Bullet Cluster
Bullet cluster
The Bullet cluster consists of two colliding clusters of galaxies. Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of dark matter...

, lensing observations show that much of the lensing mass is separated from the X-ray-emitting baryonic mass.

Galactic rotation curves



For 40 years after Zwicky's initial observations, no other corroborating observations indicated that the mass to light ratio
Mass to light ratio
In astrophysics and physical cosmology the mass to light ratio, normally designated with the symbol \Upsilon is the quotient between the total mass of a spatial volume and its luminosity...

 was anything other than unity. Then, in the late 1960s and early 1970s, Vera Rubin
Vera Rubin
Vera Rubin is an American astronomer who pioneered work on galaxy rotation rates. She is famous for uncovering the discrepancy between the predicted angular motion of galaxies and the observed motion, by studying galactic rotation curves...

, a young astronomer at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington presented findings based on a new sensitive spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...

 that could measure the velocity curve
Galaxy rotation curve
The rotation curve of a galaxy can be represented by a graph that plots the orbital velocity of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis....

 of edge-on spiral galaxies to a greater degree of accuracy than had ever before been achieved. Together with fellow staff-member Kent Ford
Kent Ford
W. Kent Ford, Jr. was an astronomer involved with the theory of dark matter. He worked with scientist Vera Rubin, who used his advanced spectrometer in her studies of space and matter. This spectrometer allowed the pair to drastically change the way dark matter was viewed, by analyzing the...

, Rubin announced at a 1975 meeting of the American Astronomical Society
American Astronomical Society
The American Astronomical Society is an American society of professional astronomers and other interested individuals, headquartered in Washington, DC...

 the discovery that most star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s in spiral galaxies
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...

 orbit at roughly the same speed, which implied that their mass densities were uniform well beyond the locations with most of the stars (the galactic bulge). An influential paper presented these results in 1980. These results suggest that either Newtonian gravity does not apply universally or that, conservatively, upwards of 50% of the mass of galaxies was contained in the relatively dark galactic halo. Met with skepticism, Rubin insisted that the observations were correct. Eventually other astronomers began to corroborate her work and it soon became well-established that most galaxies were in fact dominated by "dark matter":
  • Low Surface Brightness
    Low surface brightness galaxy
    A low surface brightness galaxy, or LSB galaxy, is a diffuse galaxy with a surface brightness that, when viewed from Earth, is at least one magnitude lower than the ambient night sky....

     (LSB) galaxies. LSBs are probably everywhere dark matter-dominated, with the observed stellar populations making only a small contribution to rotation curves. Such a property is extremely important because it allows one to avoid the difficulties associated with the deprojection and disentanglement of the dark and visible contributions to the rotation curves.
  • Spiral Galaxies
    Spiral galaxy
    A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...

    . Rotation curves of both low and high surface luminosity galaxies appear to suggest a universal density profile, which can be expressed as the sum of an exponential thin stellar disk, and a spherical dark matter halo with a flat core of radius r0 and density ρ0 = 4.5 × 10−2(r0/kpc
    Parsec
    The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

    )−2/3 Mpc
    Parsec
    The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

    −3 (here, M denotes a solar mass, 2 × 1030 kg).
  • Elliptical galaxies
    Elliptical galaxy
    An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...

    . Some elliptical galaxies show evidence for dark matter via strong gravitational lensing, X-ray evidence reveals the presence of extended atmospheres of hot gas that fill the dark haloes of isolated ellipticals and whose hydrostatic support provides evidence for dark matter. Other ellipticals have low velocities in their outskirts (tracked for example by planetary nebulae) and were interpreted as not having dark matter haloes. However simulations of disk-galaxy mergers indicate that stars were torn by tidal forces from their original galaxies during the first close passage and put on outgoing trajectories, explaining the low velocities even with a DM halo. More research is needed to clarify this situation.


Note that simulated DM haloes have significantly steeper density profiles (having central cusps) than are inferred from observations, which is a problem for cosmological models with dark matter at the smallest scale of galaxies as of 2008. This may only be a problem of resolution: star-forming regions which might alter the dark matter distribution via outflows of gas have been too small to resolve and model simultaneously with larger dark matter clumps. A recent simulation of a dwarf galaxy resolving these star-forming regions reported that strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of a galactic bulge and decreases the dark matter density to less than half of what it would have been in the central kiloparsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

. These simulation predictions—bulgeless and with shallow central dark matter profiles—correspond closely to observations of actual dwarf galaxies. There are no such discrepancies at the larger scales of clusters of galaxies and above, or in the outer regions of haloes of galaxies.

Exceptions to this general picture of DM haloes for galaxies appear to be galaxies with mass-to-light ratios close to that of stars. Subsequent to this, numerous observations have been made that do indicate the presence of dark matter in various parts of the cosmos. Together with Rubin's findings for spiral galaxies and Zwicky's work on galaxy clusters, the observational evidence for dark matter has been collecting over the decades to the point that today most astrophysicists accept its existence. As a unifying concept, dark matter is one of the dominant features considered in the analysis of structures on the order of galactic scale and larger.

Velocity dispersions of galaxies


In astronomy, the velocity dispersion
Velocity dispersion
In astronomy, the velocity dispersion σ, is the range of velocities about the mean velocity for a group of objects, such as a cluster of stars about a galaxy...

 σ, is the range of velocities about the mean velocity for a group of objects, such as a cluster of stars about a galaxy.

Rubin's pioneering work has stood the test of time. Measurements of velocity curves in spiral galaxies were soon followed up with velocity dispersion
Velocity dispersion
In astronomy, the velocity dispersion σ, is the range of velocities about the mean velocity for a group of objects, such as a cluster of stars about a galaxy...

s of elliptical galaxies
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...

. While sometimes appearing with lower mass-to-light ratios, measurements of ellipticals still indicate a relatively high dark matter content. Likewise, measurements of the diffuse interstellar gas
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

 found at the edge of galaxies indicate not only dark matter distributions that extend beyond the visible limit of the galaxies, but also that the galaxies are virialized (i.e. gravitationally bound with velocities corresponding to predicted orbital velocities of general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

) up to ten times their visible radii. This has the effect of pushing up the dark matter as a fraction of the total amount of gravitating matter from 50% measured by Rubin to the now accepted value of nearly 95%.

There are places where dark matter seems to be a small component or totally absent. Globular clusters show little evidence that they contain dark matter, though their orbital interactions with galaxies do show evidence for galactic dark matter. For some time, measurements of the velocity profile of stars seemed to indicate concentration of dark matter in the disk of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 galaxy, however, now it seems that the high concentration of baryonic matter in the disk of the galaxy (especially in the interstellar medium) can account for this motion. Galaxy mass profiles are thought to look very different from the light profiles. The typical model for dark matter galaxies is a smooth, spherical distribution in virialized halos
Dark matter halo
A dark matter halo is a hypothetical component of a galaxy, which extends beyond the edge of the visible galaxy and dominates the total mass. Since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas...

. Such would have to be the case to avoid small-scale (stellar) dynamical effects. Recent research reported in January 2006 from the University of Massachusetts Amherst
University of Massachusetts Amherst
The University of Massachusetts Amherst is a public research and land-grant university in Amherst, Massachusetts, United States and the flagship of the University of Massachusetts system...

 would explain the previously mysterious warp in the disk of the Milky Way by the interaction of the Large
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...

 and Small Magellanic Cloud
Small Magellanic Cloud
The Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....

s and the predicted 20 fold increase in mass of the Milky Way taking into account dark matter.

In 2005, astronomers from Cardiff University
Cardiff University
Cardiff University is a leading research university located in the Cathays Park area of Cardiff, Wales, United Kingdom. It received its Royal charter in 1883 and is a member of the Russell Group of Universities. The university is consistently recognised as providing high quality research-based...

 claimed to discover a galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 made almost entirely of dark matter, 50 million light years away in the Virgo Cluster
Virgo Cluster
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member...

, which was named VIRGOHI21
VIRGOHI21
VIRGOHI21 is an extended region of neutral hydrogen in the Virgo cluster discovered in 2005. Analysis of its internal motion indicates that it may contain a large amount of dark matter, as much as a small galaxy. Since VIRGOHI21 apparently contains no stars, this would make it one of the first...

. Unusually, VIRGOHI21 does not appear to contain any visible stars: it was seen with radio frequency observations of hydrogen. Based on rotation profiles, the scientists estimate that this object contains approximately 1000 times more dark matter than hydrogen and has a total mass of about 1/10 that of the Milky Way Galaxy we live in. For comparison, the Milky Way is believed to have roughly 10 times as much dark matter as ordinary matter. Models of the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 and structure formation have suggested that such dark galaxies
Dark galaxy
A dark galaxy is a hypothetical galaxy composed of dark matter. Dark galaxies receive their name because they have no detectable stars and are theoretically invisible...

 should be very common in the universe, but none had previously been detected. If the existence of this dark galaxy
Dark galaxy
A dark galaxy is a hypothetical galaxy composed of dark matter. Dark galaxies receive their name because they have no detectable stars and are theoretically invisible...

 is confirmed, it provides strong evidence for the theory of galaxy formation and poses problems for alternative explanations of dark matter.

There are some galaxies whose velocity profile indicates an absence of dark matter, such as NGC 3379.
There is evidence that there are 10 to 100 times fewer small galaxies than permitted by what the dark matter theory of galaxy formation predicts. This is known as the dwarf galaxy problem
Dwarf galaxy problem
The dwarf galaxy problem is one that arises from numerical cosmological simulations that predict the evolution of the distribution of matter in the universe. Dark matter seems to cluster hierarchically and in ever increasing number counts for smaller and smaller sized halos...

.

Galaxy clusters and gravitational lensing


A gravitational lens is formed when the light from a very distant, bright source (such as a quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

) is "bent" around a massive object (such as a cluster of galaxies
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...

) between the source object and the observer. The process is known as gravitational lensing.

Dark matter affects galaxy clusters as well. X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 measurements of hot intracluster gas correspond closely to Zwicky's observations of mass-to-light ratios for large clusters of nearly 10 to 1. Many of the experiments of the Chandra X-ray Observatory
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...

 use this technique to independently determine the mass of clusters.

The galaxy cluster Abell 2029
Abell 2029
Abell 2029 is a large cluster of galaxies a billion light years away in the constellation Virgo. The central galaxy is perhaps the largest known, with estimates ranging from 5.6 to over 6 million light years across; contrast this with the Milky Way, which is 100,000 light years across...

 is composed of thousands of galaxies enveloped in a cloud of hot gas, and an amount of dark matter equivalent to more than 1014 Suns. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies. The measured orbital velocities of galaxies within galactic clusters have been found to be consistent with dark matter observations.

Another important tool for future dark matter observations is gravitational lensing. Lensing relies on the effects of general relativity to predict masses without relying on dynamics, and so is a completely independent means of measuring the dark matter. Strong lensing, the observed distortion of background galaxies into arcs when the light passes through a gravitational lens, has been observed around a few distant clusters including Abell 1689
Abell 1689
Abell 1689 is a galaxy cluster in the constellation Virgo. It is one of the biggest and most massive galaxy clusters known and acts as a gravitational lens, distorting the images of galaxies that lie behind it...

 (pictured right). By measuring the distortion geometry, the mass of the cluster causing the phenomena can be obtained. In the dozens of cases where this has been done, the mass-to-light ratios obtained correspond to the dynamical dark matter measurements of clusters.

A technique has been developed over the last 10 years called weak gravitational lensing
Weak gravitational lensing
While the presence of any mass bends the path of light passing near it, this effect rarely produces the giant arcs and multiple images associated with strong gravitational lensing. Most lines of sight in the universe are thoroughly in the weak lensing regime, in which the deflection is impossible...

, which looks at minute distortions of galaxies observed in vast galaxy surveys
Redshift survey
In astronomy, a redshift survey, or galaxy survey, is a survey of a section of the sky to measure the redshift of astronomical objects. Using Hubble's law, the redshift can be used to calculate the distance of an object from Earth. By combining redshift with angular position data, a redshift...

 due to foreground objects through statistical analyses. By examining the apparent shear deformation of the adjacent background galaxies, astrophysicists can characterize the mean distribution of dark matter by statistical means and have found mass-to-light ratios that correspond to dark matter densities predicted by other large-scale structure measurements. The correspondence of the two gravitational lens techniques to other dark matter measurements has convinced almost all astrophysicists that dark matter actually exists as a major component of the universe's composition.

The most direct observational evidence to date for dark matter is in a system known as the Bullet Cluster
Bullet cluster
The Bullet cluster consists of two colliding clusters of galaxies. Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of dark matter...

. In most regions of the universe, dark matter and visible material are found together, as expected because of their mutual gravitational attraction. In the Bullet Cluster, a collision between two galaxy cluster
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...

s appears to have caused a separation of dark matter and baryonic matter. X-ray observations
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...

 show that much of the baryonic matter (in the form of 107–108 Kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 gas, or plasma
Astrophysical plasma
An astrophysical plasma is a plasma the physical properties of which are studied as part of astrophysics. Much of the baryonic matter of the universe is thought to consist of plasma, a state of matter in which atoms and molecules are so hot, that they have ionized by breaking up into their...

) in the system is concentrated in the center of the system. Electromagnetic interactions between passing gas particles caused them to slow down and settle near the point of impact. However, weak gravitational lensing observations of the same system show that much of the mass resides outside of the central region of baryonic gas. Because dark matter does not interact by electromagnetic forces, it would not have been slowed in the same way as the X-ray visible gas, so the dark matter components of the two clusters passed through each other without slowing down substantially. This accounts for the separation. Unlike the galactic rotation curves, this evidence for dark matter is independent of the details of Newtonian gravity, so it is held as direct evidence of the existence of dark matter. Another galaxy cluster, known as the Train Wreck Cluster/Abell 520, seems to have its dark matter completely separated from both the galaxies and the gas in that cluster, which presents some problems for theoretical models.

Cosmic microwave background


The discovery and confirmation of the cosmic microwave background (CMB) radiation occurred in 1964.
Since then, many further measurements of the CMB have also supported and constrained this theory, perhaps the most famous being the NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 Cosmic Background Explorer (COBE
COBE
The COsmic Background Explorer , also referred to as Explorer 66, was a satellite dedicated to cosmology. Its goals were to investigate the cosmic microwave background radiation of the universe and provide measurements that would help shape our understanding of the cosmos.This work provided...

). COBE found a residual temperature of 2.726 K and in 1992 detected for the first time the fluctuations (anisotropies) in the CMB, at a level of about one part in 105. During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. The primary goal of these experiments was to measure the angular scale of the first acoustic peak of the power spectrum of the anisotropies, for which COBE did not have sufficient resolution. In 2000–2001, several experiments, most notably BOOMERanG
Boomerang
A boomerang is a flying tool with a curved shape used as a weapon or for sport.-Description:A boomerang is usually thought of as a wooden device, although historically boomerang-like devices have also been made from bones. Modern boomerangs used for sport are often made from carbon fibre-reinforced...

 found the Universe to be almost spatially flat by measuring the typical angular size (the size on the sky) of the anisotropies. During the 1990s, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment reported that the highest power fluctuations occur at scales of approximately one degree. These measurements were able to rule out cosmic strings as the leading theory of cosmic structure formation
Structure formation
Structure formation refers to a fundamental problem in physical cosmology. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.7 Gyr ago...

, and suggested cosmic inflation
Cosmic inflation
In physical cosmology, cosmic inflation, cosmological inflation or just inflation is the theorized extremely rapid exponential expansion of the early universe by a factor of at least 1078 in volume, driven by a negative-pressure vacuum energy density. The inflationary epoch comprises the first part...

 was the right theory.

A number of ground-based interferometers provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array
Very Small Array
The Very Small Array is a 14-element interferometric radio telescope operating between 26 and 36 GHz that is used to study the cosmic microwave background radiation. It is a collaboration between the University of Cambridge, University of Manchester and the Instituto de Astrofisica de Canarias...

, Degree Angular Scale Interferometer
Degree Angular Scale Interferometer
The Degree Angular Scale Interferometer was a telescope located in Antarctica. It was a 13-element interferometer operating between 26 and 36 GHz in ten bands. The instrument is similar in design to the Cosmic Background Imager and the Very Small Array...

 (DASI) and the Cosmic Background Imager
Cosmic Background Imager
The Cosmic Background Imager was a 13-element interferometer perched at an elevation of 5,080 metres at Llano de Chajnantor Observatory in the Chilean Andes...

 (CBI). DASI made the first detection of the polarization of the CMB
and the CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum. COBE's successor, the Wilkinson Microwave Anisotropy Probe
Wilkinson Microwave Anisotropy Probe
The Wilkinson Microwave Anisotropy Probe — also known as the Microwave Anisotropy Probe , and Explorer 80 — is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat — the Cosmic Microwave Background Radiation — across the full sky. Headed by Professor...

 (WMAP) has provided the most detailed measurements of (large-scale)anisotropies in the CMB as of 2009. WMAP's measurements played the key role in establishing the current Standard Model of Cosmology, namely the Lambda-CDM model
Lambda-CDM model
ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy...

, a flat universe dominated by dark energy, supplemented by dark matter
and atoms with density fluctuations seeded by a Gaussian
GAUSSIAN
Gaussian is a computational chemistry software program initially released in 1970 by John Pople and his research group at Carnegie-Mellon University as Gaussian 70. It has been continuously updated since then...

, adiabatic, nearly scale invariant
process. The basic properties of this universe are determined by five numbers: the density of
matter, the density of atoms, the age of the universe (or equivalently, the Hubble constant
today), the amplitude of the initial fluctuations, and their scale dependence. This model also requires a period of cosmic inflation. The WMAP data in fact ruled out several more complex cosmic inflation models, though supporting the one in Lambda-CDM amongst others.

In summary, a successful Big Bang cosmology theory must fit with all available astronomical observations (known as the concordance model), in particular the CMB. In cosmology the CMB is explained as relic radiation from the big bang, originally at thousands of degrees kelvin but red shift
Red shift
-Science:* Redshift, the increase of wavelength of detected electromagnetic radiation with respect to the original wavelength of the emission* Red shift, an informal term for a bathochromic shift...

ed down to microwave by the expansion of the universe over the last thirteen billion years. The anisotropies in the CMB are explained as acoustic oscillations in the photon-baryon plasma (prior to the emission of the CMB after the photons decouple from the baryons at 379,000 years after the Big Bang) whose restoring force is gravity. Ordinary (baryonic) matter interacts strongly with radiation whereas, by definition, dark matter does not—though both affect the oscillations by their gravity—so the two forms of matter will have different effects. The power spectrum of the CMB anisotropies shows a large main peak and smaller successive peaks, resolved down to the third peak as of 2009.e.g.. The main peak tells you most about the density of baryonic matter and the third peak most about the density of dark matter (see Cosmic microwave background radiation#Primary anisotropy).

Sky Surveys and Baryon Acoustic Oscillations



The acoustic oscillations in the early universe (see the previous section) leave their imprint in the visible matter by Baryon Acoustic Oscillation (BAO) clustering, in a way that can be measured with sky surveys such as the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...

 and the 2dF Galaxy Redshift Survey
2dF Galaxy Redshift Survey
In astronomy, the 2dF Galaxy Redshift Survey , 2dF or 2dFGRS is a redshift survey conducted by the Anglo-Australian Observatory with the 3.9m Anglo-Australian Telescope between 1997 and 11 April 2002. The data from this survey were made public on 30 June 2003...

. These measurements are consistent with those of the CMB derived from the WMAP spacecraft and further constrain the Lambda CDM model and dark matter. Note that the CMB data and the BAO data measure the acoustic oscillations at very different distance scales
Cosmic distance ladder
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth...

.

Type Ia supernovae distance measurements



Type Ia supernovae can be used as "standard candles
Standard Candles
Standard Candles is a compilation of short stories by American science fiction author Jack McDevitt. The sixteen stories in the anthology were originally published in various magazines from 1982 to 1996...

" to measure extragalactic distances, and extensive data sets of these supernovae can be used to constrain cosmological models. They constrain the dark energy density ΩΛ= ~0.713 for a flat, Lambda CDM Universe and the parameter w for a quintessence
Quintessence (physics)
In physics, quintessence is a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. It has been proposed by some physicists to be a fifth fundamental force...

 model. Once again, the values obtained are roughly consistent with those derived from the WMAP observations and further constrain the Lambda CDM model and (indirectly) dark matter.

Lyman-alpha forest



In astronomical spectroscopy
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...

, the Lyman-alpha forest is the sum of absorption line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

s arising from the Lyman-alpha
Lyman series
In physics and chemistry, the Lyman series is the series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1...

 transition of the neutral hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 in the spectra of distant galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 and quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

s.
Observations of the Lyman-alpha forest can also be used to constrain cosmological models. These constraints are again in agreement with those obtained from WMAP data.

Structure formation


Dark matter is crucial to the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 model of cosmology as a component which corresponds directly to measurements of the parameters
Lambda-CDM model
ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy...

 associated with Friedmann cosmology solutions to general relativity. In particular, measurements of the cosmic microwave background anisotropies correspond to a cosmology where much of the matter interacts with photons more weakly than the known forces that couple light interactions to baryonic matter. Likewise, a significant amount of non-baryon
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...

ic, cold matter is necessary to explain the large-scale structure of the universe.

Observations suggest that structure formation
Structure formation
Structure formation refers to a fundamental problem in physical cosmology. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.7 Gyr ago...

 in the universe proceeds hierarchically, with the smallest structures collapsing first and followed by galaxies and then clusters of galaxies. As the structures collapse in the evolving universe, they begin to "light up" as the baryonic matter heats up through gravitational contraction and the object approaches hydrostatic pressure balance
Hydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...

. Ordinary baryonic matter had too high a temperature, and too much pressure left over from the Big Bang to collapse and form smaller structures, such as stars, via the Jeans instability
Jeans instability
In physics, the Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation. It occurs when the internal gas pressure is not strong enough to prevent gravitational collapse of a region filled with matter...

. Dark matter acts as a compactor of structure. This model not only corresponds with statistical surveying of the visible structure in the universe but also corresponds precisely to the dark matter predictions of the cosmic microwave background. However, in detail, some issues remain yet to be addressed including an absence of satellite galaxies from simulations
Dwarf galaxy problem
The dwarf galaxy problem is one that arises from numerical cosmological simulations that predict the evolution of the distribution of matter in the universe. Dark matter seems to cluster hierarchically and in ever increasing number counts for smaller and smaller sized halos...

 and cores of dark matter halos which appear smoother than predicted
Cuspy halo problem
The cuspy halo problem arises from cosmological simulations that seem to indicate cold dark matter would form cuspy distributions — that is, increasing sharply to a high value at a central point — in the most dense areas of the universe. This would imply that the center of our galaxy,...

.

This bottom up model of structure formation requires something like cold dark matter to succeed. Large computer simulations of billions of dark matter particles have been used to confirm that the cold dark matter model of structure formation is consistent with the structures observed in the universe through galaxy surveys, such as the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...

 and 2dF Galaxy Redshift Survey
2dF Galaxy Redshift Survey
In astronomy, the 2dF Galaxy Redshift Survey , 2dF or 2dFGRS is a redshift survey conducted by the Anglo-Australian Observatory with the 3.9m Anglo-Australian Telescope between 1997 and 11 April 2002. The data from this survey were made public on 30 June 2003...

, as well as observations of the Lyman-alpha forest
Lyman-alpha forest
In astronomical spectroscopy, the Lyman-alpha forest is the sum of absorption lines arising from the Lyman-alpha transition of the neutral hydrogen in the spectra of distant galaxies and quasars....

. These studies have been crucial in constructing the Lambda-CDM model
Lambda-CDM model
ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy...

 which measures the cosmological parameters, including the fraction of the universe made up of baryons and dark matter.

History of the search for its composition



Although dark matter had historically been inferred by many astronomical observations, its composition long remained speculative. Early theories of dark matter concentrated on hidden heavy normal objects, such as black holes, neutron stars, faint old white dwarfs, brown dwarfs, as the possible candidates for dark matter, collectively known as MACHOs
Machos
Machos is a community in western Elis in Greece. The population is around 200. Its dialing code is 26230...

. Astronomical surveys failed to find enough of these hidden MACHOs. Some hard-to-detect baryonic matter
Baryonic dark matter
In astronomy and cosmology, baryonic dark matter is dark matter composed of baryons, i.e. protons and neutrons and combinations of these, such as non-emitting ordinary atoms...

, such as MACHOs and some forms of gas, additionally were believed to make a contribution to the overall dark matter content but evidence indicated such would constitute only a small portion.

Furthermore, data from a number of lines of other evidence, including galaxy rotation curves, gravitational lensing, structure formation, and the fraction of baryons in clusters and the cluster abundance combined with independent evidence for the baryon density, indicated that 85–90% of the mass in the universe does not interact with the electromagnetic force. This "nonbaryonic dark matter" is evident through its gravitational effect. Consequently, the most commonly held view was that dark matter is primarily non-baryonic, made of one or more elementary particles other than the usual electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s, proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s, neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s, and known neutrinos. The most commonly proposed particles then became axion
Axion
The axion is a hypothetical elementary particle postulated by the Peccei-Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics...

s, sterile neutrinos, and WIMPs (Weakly Interacting Massive Particles, including neutralino
Neutralino
In particle physics, the neutralino is a hypothetical particle predicted by supersymmetry. There are four neutralinos that are fermions and are electrically neutral, the lightest of which is typically stable...

s).

The dark matter component has much more mass than the "visible" component of the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

. Only about 4.6% of the mass of the Universe is ordinary matter. About 23% is thought to be composed of dark matter. The remaining 72% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

. It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance, much like the marking of early maps with "terra incognita
Terra incognita
Terra incognita or terra ignota is a term used in cartography for regions that have not been mapped or documented. The expression is believed to be first seen in Ptolemy’s Geography circa 150 CE...

".

Historically, three categories of dark matter candidates had been postulated. The categories cold, warm, and hot refer to the speed at which the particles are traveling rather than an actual temperature.
  • Cold dark matter
objects that move at classical velocities
  • Warm dark matter
particles that move relativistically
  • Hot dark matter
particles that move ultrarelativistically
Ultrarelativistic limit
In physics, a particle is called ultrarelativistic when its speed is very close to the speed of light c.Max Planck showed that the relativistic expression for the energy of a particle whose rest mass is m and momentum is p is given by E^2 = m^2 c^4 + p^2 c^2...



Though a fourth category had been considered early on, called mixed dark matter
Mixed dark matter
A theory of dark matter which was promising up to the late 1990s."Mixed" dark matter is also called Hot + Cold Dark Matter. Hot Dark matter has only one known form - neutrinos, although other forms are speculated to exist. Cosmologically important dark matter is now believed to be pure Cold Dark...

, it was quickly eliminated (from the 1990s) since the discovery of dark energy.

Mixed dark matter



Mixed dark matter is a now obsolete model, with a specifically chosen mass ratio of 80% cold dark matter and 20% hot dark matter (neutrinos) content. Though it is presumable that hot dark matter coexists with cold dark matter in any case, there was a very specific reason for choosing this particular ratio of hot to cold dark matter in this model. This model was promising until the late 1990s, when it was superseded by the Dark Energy model, with the discovery of Dark energy. Prior to the discovery of Dark energy, this model was a good fit for the cosmic microwave background spectrum fluctuation data that were just coming in at that time. The highly relativistic hot dark matter (i.e. neutrinos) took the place of the yet-to-be-discovered dark energy within the observed mixed dark matter
Mixed dark matter
A theory of dark matter which was promising up to the late 1990s."Mixed" dark matter is also called Hot + Cold Dark Matter. Hot Dark matter has only one known form - neutrinos, although other forms are speculated to exist. Cosmologically important dark matter is now believed to be pure Cold Dark...

 spectrum, at that time referred to as the "fluctuation spectrum." As an example, Davis et al. wrote in 1985:

Cold dark matter



Today, the "cold" argument wins popular scientific acceptance for explaining observable phenomena, in particular the most commonly accepted theory today is that dark matter appears to be composed solely of weakly interacting massive particles (WIMPs).

Cold Dark Matter is dark matter traveling at classical (non-relativistic) speeds. Generally, this is less than 0.1c. This is currently the area of greatest interest for dark matter research, as hot and warm dark matter are not viable theories for galaxy and galaxy cluster formation.

The Concordance model requires that, to explain structure in the universe, it is necessary to invoke cold dark matter. What this cold dark matter can be is completely flexible. They can be large objects like MACHOs
Machos
Machos is a community in western Elis in Greece. The population is around 200. Its dialing code is 26230...

 or RAMBOs
Robust Associations of Massive Baryonic Objects
In astronomy, a RAMBO or robust association of massive baryonic objects is a dark cluster made of brown dwarfs or white dwarfs.-Introduction:RAMBOs were proposed by Moore and Silk in 1995...

, or particles like WIMPs, axions, dark matter "X-particles" such as Holeum
Holeum
Holeums are stable, quantized gravitational bound states of primordial or micro black holes.-Introduction:Holeums were proposed by L.K. Chavda and Abhijit Chavda in 2002. They have all the properties associated with cold dark matter...

s etc.

Large masses, like galaxy-sized black holes can be ruled out on the basis of gravitational lensing data. However, many primordial
Primordial black hole
A primordial black hole is a hypothetical type of black hole that is formed not by the gravitational collapse of a large star but by the extreme density of matter present during the universe's early expansion....

 (created in the big bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 instead of by mass accretion) intermediate mass black holes between 30 and 300,000 solar masses in galactic halos are consistent with observations of wide binaries
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

 as well as microlensing and galactic disk stability. Other possibilities involving normal baryonic matter include brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

s or perhaps small, dense chunks of heavy elements; such objects are known as massive compact halo object
Massive compact halo object
Massive astrophysical compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter, which emits little or no radiation and drifts through interstellar...

s, or "MACHOs". However, studies of big bang nucleosynthesis
Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of H-1 during the early phases of the universe...

 have convinced most scientists that non-primordial baryonic matter such as MACHOs cannot be more than a small fraction of the total dark matter.

The DAMA/NaI
DAMA/NaI
The DAMA/NaI experiment was designed to detect dark matter using the direct detection technique. It was located at the Laboratori Nazionali del Gran Sasso in Italy and collected data during the period 1996-2002...

 experiment and its successor DAMA/LIBRA
DAMA/LIBRA
The DAMA/LIBRA experiment is an experiment designed to detect dark matter using the direct detection technique. It is located at the Laboratori Nazionali del Gran Sasso in Italy. The successor to the DAMA/NaI experiment, it uses very similar detector technology but has a larger target mass of...

 have claimed to directly detect dark matter passing through the Earth, though many scientists remain skeptical since negative results of other experiments are (almost) incompatible with the DAMA results if dark matter consists of neutralino
Neutralino
In particle physics, the neutralino is a hypothetical particle predicted by supersymmetry. There are four neutralinos that are fermions and are electrically neutral, the lightest of which is typically stable...

s. Another view is that the DAMA results are evidence that neutralino
Neutralino
In particle physics, the neutralino is a hypothetical particle predicted by supersymmetry. There are four neutralinos that are fermions and are electrically neutral, the lightest of which is typically stable...

s might not constitute dark matter, so that scientists should get to work on finding dark matter theories consistent with the experiments.

None of these are part of the standard model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

 of particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

, but they can arise in extensions to the standard model. In many supersymmetric
Supersymmetry
In particle physics, supersymmetry is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartners...

 models naturally give rise to stable dark matter candidates in the form of the Lightest Supersymmetric Particle
Lightest Supersymmetric Particle
In particle physics, the Lightest Supersymmetric Particle is the generic name given to the lightest of the additional hypothetical particles found in supersymmetric models. In models with R-parity conservation, the LSP is stable. There is extensive observational evidence for an additional...

 (LSP); a neutralino is an example of a Supersymmetric particle. Separately, heavy sterile neutrinos exist in non-supersymmetric extensions to the standard model that explain the small neutrino mass through the seesaw mechanism
Seesaw mechanism
In theoretical physics, the seesaw mechanism is a mechanism within grand unification theory, and in particular in theories of neutrino masses and neutrino oscillation, where it can be used to explain the smallness of observed neutrino masses relative to those of quarks and leptons.There are several...

.

Warm dark matter



Warm dark matter are particles traveling at relativistic speeds, but less than ultra-relativistic speeds. This is typically interpreted as a velocity range of 0.1c to 0.95c.

Neither hot nor warm dark matter can explain how individual galaxies formed from the Big Bang. That is because hot and warm dark matter move too quickly to be bound to galaxies and thus explain the traditional problems of galactic rotational curves and velocity dispersions that dark matter was postulated to address in the first place. Likewise, hot and warm dark matter moves too quickly to stay together to form the larger-scale structures that can be observed that form weak gravitational lenses (e.g. galaxy clusters).

The microwave background radiation while incredibly smooth, has tiny temperature fluctuations which indicate that matter had clumped on very small scales, which then grew to become the huge galactic clusters and voids seen in the universe today. Fast moving particles, however, cannot clump together on such small scales and, in fact, suppress the clumping of other matter.

There have been no particles discovered so far that can be categorized as warm dark matter. There is a postulated candidate for the warm dark matter category, which is the sterile neutrino
Sterile neutrino
Sterile neutrinosIn scientific literature, these particles are also variously referred to as right-handed neutrinos, inert neutrinos, heavy neutrinos, or neutral heavy leptons . are a hypothetical type of neutrino that do not interact via any of the fundamental interactions of the Standard Model...

: a heavier, slower form of neutrino which does not even interact through the Weak force unlike regular neutrinos. If warm dark matter particles do exist, it would not be enough to explain galactic formation, and cold dark matter would still be required to fill that purpose. Interestingly, some modified gravity theories, such as Scalar-tensor-vector gravity
Scalar-tensor-vector gravity
Scalar–tensor–vector gravity is a modified theory of gravity developed by John Moffat, a researcher at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario...

, also require that a warm dark matter exist to make their equations work out.

Hot dark matter



Hot dark matter are particles that travel at ultra-relativistic
Ultrarelativistic limit
In physics, a particle is called ultrarelativistic when its speed is very close to the speed of light c.Max Planck showed that the relativistic expression for the energy of a particle whose rest mass is m and momentum is p is given by E^2 = m^2 c^4 + p^2 c^2...

 velocities. These are approximately velocities over 0.95c.

An example of hot dark matter is already known: the neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...

. Neutrinos were discovered quite separately from, and long before the search for dark matter seriously began: they were first postulated in 1930, and first detected in 1956. Neutrinos have a very small mass: at least 100,000 times less massive than an electron. Other than gravity, neutrinos only interact with normal matter via the weak force
Weak interaction
Weak interaction , is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars...

 making them very difficult to detect (the weak force only works over a small distance, thus a neutrino will only trigger a weak force event if it hits a nucleus directly head-on). This would classify them as Weakly Interacting, Light Particles, or WILPs, as opposed to cold dark matter's theoretical candidates, the WIMPs.

There are three different known flavors of neutrinos (i.e. the electron-, muon-, and tau-neutrinos), and their masses are slightly different. The resolution to the solar neutrino problem
Solar neutrino problem
The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrinos flowing through the Earth and theoretical models of the solar interior, lasting from the mid-1960s to about 2002...

 demonstrated that these three types of neutrinos actually change and oscillate from one flavor to the others and back as they are in-flight. It's hard to determine an exact upper bound
Upper bound
In mathematics, especially in order theory, an upper bound of a subset S of some partially ordered set is an element of P which is greater than or equal to every element of S. The term lower bound is defined dually as an element of P which is lesser than or equal to every element of S...

 on the collective average mass of the three neutrinos (let alone a mass for any of the three individually). For example, if the average neutrino mass were chosen to be over 50  (which is still over 10,000 times less massive than an electron), just by the sheer number of them in the universe, the universe would collapse due to their mass. So other observations have served to estimate an upper-bound for the neutrino mass. Using cosmic microwave background data and other methods, it is currently believed that their average mass probably does not exceed 0.3 . Thus, the normal forms of neutrinos cannot be responsible for the measured dark matter component from cosmology.

Most gravitational lensing data usually gets explained through cold dark matter theories. Nevertheless, at least one example of lensing data, that of galaxy cluster Abell 1689
Abell 1689
Abell 1689 is a galaxy cluster in the constellation Virgo. It is one of the biggest and most massive galaxy clusters known and acts as a gravitational lens, distorting the images of galaxies that lie behind it...

, can be supported by a light fermionic dark matter in the mass range of few ; in particular: neutrinos with a mass of about 1.5 . In this model-fit, active (left-handed
Chirality (physics)
A chiral phenomenon is one that is not identical to its mirror image . The spin of a particle may be used to define a handedness for that particle. A symmetry transformation between the two is called parity...

) neutrinos account for some 9.5% dark matter with as yet unobserved sterile
Sterile neutrino
Sterile neutrinosIn scientific literature, these particles are also variously referred to as right-handed neutrinos, inert neutrinos, heavy neutrinos, or neutral heavy leptons . are a hypothetical type of neutrino that do not interact via any of the fundamental interactions of the Standard Model...

 (right-handed) ones accounting for the rest.

Hot dark matter travels too quickly to be bound by an individual galaxy or a galaxy cluster's gravity, though a heavy neutrino might be able to affect the shapes of the even larger structures like galaxy superclusters. Thus, hot dark matter is not enough to explain how galaxies form and stay the way they are (e.g. rotation curves). Therefore they would only form a part of the story, and a cold dark matter candidate would still need to be found. Certain theories of modified gravity, such as TeVeS
Teves
Teves may refer to* Continental Teves, an automotive brake component manufacturer based in Wales, now part of Continental AG* TVes, a Venezuelan TV station * Tensor-vector-scalar gravity theory by Jacob Bekenstein...

 still require neutrino hot dark matter with a certain mass range to make their equations work.

Composition


An important property of all dark matter is that it behaves like and is modeled like a perfect fluid
Perfect fluid
In physics, a perfect fluid is a fluid that can be completely characterized by its rest frame energy density ρ and isotropic pressure p....

, meaning that it does not have any internal resistance or viscosity. This means that dark matter particles should not interact with each other (except through gravity), i.e. they move past each other without ever bumping or colliding. Also as discussed above, "cold" theories, as opposed to the "warm" or "hot" perspectives on the composition of dark matter, gained favor at better explaining observable phenomena.

Detection


If the dark matter within our galaxy is made up of Weakly Interacting Massive Particles (WIMPs), then a large number must pass through the Earth each second. There are many experiments currently running, or planned, aiming to test this hypothesis by searching for WIMPs. Although WIMPs are a more popular dark matter candidate, there are also experiments searching for other particle candidates such as axions. It is also possible that dark matter consists of very heavy hidden sector
Hidden sector
In particle physics, the term hidden sector refers to the collection of yet-unobserved quantum fields and the corresponding hypothetical particles that do not directly interact via the gauge boson forces of the Standard Model. Most typically, hidden sectors include a gauge group that is independent...

 particles which only interact with ordinary matter via gravity.

These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of WIMP annihilations.

An alternative approach to the detection of WIMPs in nature is to produce them in the laboratory. Experiments with the Large Hadron Collider
Large Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....

 (LHC) may be able to detect WIMPs produced in collisions of the LHC proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

 beams; because a WIMP has negligible interactions with matter, it may be detected indirectly as (large amounts of) missing energy and momentum which escape the LHC detectors, provided all the other (non-negligible) collision products are detected. These experiments could show that WIMPs can be created, but it would still require a direct detection experiment to show that they exist in sufficient numbers in the galaxy, to account for dark matter.

Direct detection experiments


Direct detection experiments typically operate in deep underground laboratories to reduce the background from cosmic rays. These include: the Soudan mine; the SNOLAB
Snolab
SNOLAB is a Canadian underground physics laboratory at a depth of 2 km in Sudbury, Ontario in Vale's Creighton nickel mine. The original Sudbury Neutrino Observatory experiment has ended, but the facilities have been expanded into a permanent underground laboratory.SNOLAB is the world's...

 underground laboratory at Sudbury, Ontario (Canada); the Gran Sasso National Laboratory (Italy); the Boulby Underground Laboratory
Boulby Mine
Boulby Mine is a site run by Cleveland Potash, located just southeast of the village of Boulby, on the northeast coast of the North Yorkshire Moors in Redcar and Cleveland, England. It is Europe's second deepest mine at —Pyhäsalmi Mine in Finland is even deeper, being 1,448 metres deep—producing...

 (UK); and the Deep Underground Science and Engineering Laboratory
Deep Underground Science and Engineering Laboratory
The Deep Underground Science and Engineering Laboratory, or DUSEL is a major project under consideration by the National Science Foundation. DUSEL will be a series of large laboratories, caverns, and cleanrooms serving the field of underground science...

, South Dakota (US).

The majority of present experiments use one of two detector technologies: cryogenic detectors, operating at temperatures below 100mK, detect the heat produced when a particle hits an atom in a crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

 absorber such as germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....

. Noble liquid
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...

 detectors detect the flash of scintillation
Scintillation (physics)
Scintillation is a flash of light produced in a transparent material by an ionization event. See scintillator and scintillation counter for practical applications.-Overview:...

 light produced by a particle collision in liquid xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...

 or argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

. Cryogenic detector experiments include: CDMS
Cryogenic Dark Matter Search
The Cryogenic Dark Matter Search is a series of experiments designed to directly detect particle dark matter in the form of WIMPs. Using an array of semiconductor detectors at millikelvin temperatures, CDMS has set the most sensitive limits to date on the interactions of WIMP dark matter with...

, CRESST, EDELWEISS
EDELWEISS
EDELWEISS is a dark matter search experiment located in the Modane Underground Laboratory. The experiment uses cryogenic detectors, measuring both the phonon and ionization signals produced by particle interactions in germanium crystals...

, EURECA. Noble liquid experiments include ZEPLIN, XENON
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...

, DEAP
DEAP
DEAP is a direct dark matter search experiment using liquid argon as target material. DEAP utilizes background discrimination based on the characteristic scintillation pulse shape in argon...

, ArDM
ArDM
ArDM is a particle physics experiment based on a liquid argon detector, aiming at measuring signals from WIMPs , which probably constitute the Dark Matter in the universe...

, WARP
WIMP Argon Programme
The WIMP Argon Programme is an experiment at Laboratori Nazionali del Gran Sasso, Italy, for the research of cold dark matter. It aims to detect nuclear recoils in liquid argon induced by weakly interacting massive particles through scintillation light; the apparatus can also detect ionization so...

 and LUX
Large Underground Xenon Detector
The Large Underground Xenon Detector is a 350 kg two-phase liquid xenon detector of dark matter particles. Liquid xenon both scintillates and becomes ionized when hit by particles . The ratio of scintillation over ionization energy caused by the collision provides a way of identifying the...

. Both of these detector techniques are capable of distinguishing background particles which scatter off electrons, from dark matter particles which scatter off nuclei. Other experiments include SIMPLE
SIMPLE (dark matter)
SIMPLE is an experiment search for direct evidence of dark matter. It is located in a 61 m3 cavern at the 500 level of the Laboratoire Souterrain à Bas Bruit near Apt in southern France...

 and PICASSO
PICASSO (dark matter)
The PICASSO ' experiment is an experiment searching for direct evidence of dark matter...

.

The DAMA/NaI
DAMA/NaI
The DAMA/NaI experiment was designed to detect dark matter using the direct detection technique. It was located at the Laboratori Nazionali del Gran Sasso in Italy and collected data during the period 1996-2002...

, DAMA/LIBRA
DAMA/LIBRA
The DAMA/LIBRA experiment is an experiment designed to detect dark matter using the direct detection technique. It is located at the Laboratori Nazionali del Gran Sasso in Italy. The successor to the DAMA/NaI experiment, it uses very similar detector technology but has a larger target mass of...

 experiments have detected an annual modulation in the event rate, which they claim is due to dark matter particles. (As the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount depending on the time of year). This claim is so far unconfirmed and difficult to reconcile with the negative results of other experiments assuming that the WIMP scenario is correct.

Directional detection of dark matter is a search strategy based on the motion of the Solar System around the galactic center.
By using a low pressure TPC
Time projection chamber
In physics, a time projection chamber is a particle detector invented by David R. Nygren, an American physicist, at Lawrence Berkeley Laboratory in the late 1970s...

, it is possible to access information on recoiling tracks (3D reconstruction if possible) and to constrain the WIMP-nucleus kinematics. WIMPs coming from the direction in which the Sun is travelling (roughly in the direction of the Cygnus constellation) may then be separated from background noise, which should be isotropic. Directional dark matter experiments include DMTPC, DRIFT
Directional Recoil Identification From Tracks
The Directional Recoil Identification From Tracks detector is a low pressure negative ion time projection chamber designed to detect weakly interacting massive particles - a prime dark matter candidate....

, Newage and MIMAC.

On 17 December 2009 CDMS researchers reported two possible WIMP candidate events. They estimate that the probability that these events are due to a known background (neutrons or misidentified beta or gamma events) is 23%, and conclude "this analysis cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject either event as signal."

More recently, on 4 September 2011, researchers using the CRESST detectors presented evidence of 67 collisions occurring in detector crystals from sub-atomic particles, calculating there is a less than 1 in 10,000 chance that all were caused by known sources of interference or contamination. It is quite possible then that many of these collisions were caused by WIMPs, and/or other unknown particles.

Indirect detection experiments


Indirect detection experiments search for the products of WIMP annihilation. If WIMPs are Majorana particles
Majorana fermion
In physics, a Majorana fermion is a fermion which is its own anti-particle. The term is used in opposition to Dirac fermion, which describes particles that differ from their antiparticles...

 (the particle and antiparticle are the same) then two WIMPs colliding would annihilate to produce gamma rays, and particle-antiparticle pairs. This could produce a significant number of gamma rays, antiproton
Antiproton
The antiproton is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy....

s or positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...

s in the galactic halo. The detection of such a signal is not conclusive evidence for dark matter, as the backgrounds from other sources are not fully understood.

The EGRET
Egret
An egret is any of several herons, most of which are white or buff, and several of which develop fine plumes during the breeding season. Many egrets are members of the genera Egretta or Ardea which contain other species named as herons rather than egrets...

 gamma ray telescope observed an excess of gamma rays, but scientists concluded that this was most likely a systematic effect. The Fermi Gamma-ray Space Telescope, launched June 11, 2008, is searching for gamma ray events from dark matter annihilation. At higher energies, the ground-based MAGIC
MAGIC (telescope)
MAGIC is a system of two Imaging Atmospheric Cherenkov telescopes situated at the Roque de los Muchachos Observatory on La Palma, one of the Canary Islands, at about 2200 m above sea level...

 gamma-ray telescope has set limits to the existence of dark matter in dwarf spheroidal galaxies
and clusters of galaxies.

The PAMELA payload (launched 2006) has detected an excess of positrons, which could be produced by dark matter annihilation, but may also come from pulsars. No excess of anti-protons has been observed.

WIMPs passing through the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 or Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 are likely to scatter off atoms and lose energy. This way a large population of WIMPs may accumulate at the center of these bodies, increasing the chance that two will collide and annihilate. This could produce a distinctive signal in the form of high energy neutrinos originating from the center of the Sun or Earth. It is generally considered that the detection of such a signal would be the strongest indirect proof of WIMP dark matter. High energy neutrino telescopes such as AMANDA
Antarctic Muon And Neutrino Detector Array
The Antarctic Muon And Neutrino Detector Array is a neutrino telescope located beneath the Amundsen-Scott South Pole Station. In 2005, after nine years of operation, AMANDA officially became part of its successor project, the IceCube Neutrino Observatory.AMANDA consists of optical modules, each...

, IceCube and ANTARES
ANTARES (telescope)
ANTARES is the name of a neutrino detector residing 2.5 km under the Mediterranean Sea off the coast of Toulon, France. It is designed to be used as a directional Neutrino Telescope to locate and observe neutrino flux from cosmic origins in the direction of the Southern Hemisphere of the...

 are searching for this.

WIMP annihilation from the Milky Way Galaxy as a whole may also be detected in the form of various annihilation products. The Galactic Center is a particularly good place to look as it contains the largest dark matter abundance.

Alternative theories


Although dark matter is the most popular theory to explain the various astronomical observations of galaxies and galaxy clusters, there has been no direct observational evidence of dark matter. Some alternative theories have been proposed to explain these observations without the need for a vast amount of undetected matter. They broadly fall into the categories of modified gravity laws, and quantum gravity laws. The difference between modified gravity laws and quantum gravity laws is that modified gravity laws simply propose alternative behaviour of gravity at astrophysical and cosmological scales, without any regard to the quantum scale. Both posit that gravity behaves differently at different scales of the universe, making the laws established by Newton and Einstein insufficient.

Modified gravity laws


One group of alternative theories to dark matter assume that the observed inconsistencies are due to an incomplete understanding of gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...

 rather than invisible matter. These theories propose to modify the laws of gravity instead.

The earliest modified gravity model to emerge was Mordehai Milgrom
Mordehai Milgrom
Mordehai Milgrom is an Israeli physicist and professor in the department of Condensed Matter Physics at the Weizmann Institute in Rehovot, Israel. He is most famous for his proposal of Modified Newtonian dynamics as an alternative to the dark matter and galaxy rotation curve problems, in 1981...

's Modified Newtonian Dynamics
Modified Newtonian dynamics
In physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...

 (MOND) in 1983, which adjusts Newton's laws to create a stronger gravitational field when gravitational acceleration levels become tiny (such as near the rim of a galaxy). It had some success in predicting galactic-scale features, such as rotational curves of elliptical galaxies, and dwarf elliptical galaxies, etc. It fell short in predicting galaxy cluster lensing. However, MOND was not relativistic
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

, since it was just a straight adjustment of the older Newtonian account of gravitation, not of the newer account in Einstein's general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

. Work began soon after to make MOND conform to General Relativity. It's an ongoing process, and many competing theories have emerged based around the original MOND theory, such as TeVeS
Teves
Teves may refer to* Continental Teves, an automotive brake component manufacturer based in Wales, now part of Continental AG* TVes, a Venezuelan TV station * Tensor-vector-scalar gravity theory by Jacob Bekenstein...

, and MOG or STV gravity, phenomenological covariant approach, etc.

In 2007, John W. Moffat proposed a modified gravity theory based on the Nonsymmetric Gravitational Theory
Nonsymmetric gravitational theory
In theoretical physics, the nonsymmetric gravitational theory of John Moffat is a classical theory of gravitation which tries to explain the observation of the flat rotation curves of galaxies....

 (NGT) that claims to account for the behavior of colliding galaxies. This theory still requires the presence of non-relativistic neutrinos, or other candidates for (cold) dark matter, to work.

Another proposal utilizes a gravitational backreaction in an emerging theoretical field that seeks to explain gravity between objects as an action, a reaction, and then a back-reaction. Simply, an object A affects an object B, and the object B then re-affects object A, and so on: creating somewhat of a feedback loop that strengthens gravity.

Recently, another group has proposed a modification of large scale gravity in a theory named "dark fluid
Dark fluid
In astronomy and cosmology, dark fluid is an alternative theory to both dark matter and dark energy and attempts to explain both phenomena in a single framework....

". In this formulation, the attractive gravitational effects attributed to dark matter are instead a side-effect of dark energy. Dark fluid combines dark matter and dark energy in a single energy field that produces different effects at different scales. This treatment is a simplified approach to a previous fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

-like model called the Generalized Chaplygin gas model where the whole of spacetime is a compressible gas. Dark fluid can be compared to an atmospheric system. Atmospheric pressure causes air to expand, but part of the air can collapse to form clouds. In the same way, the dark fluid might generally expand, but it also could collect around galaxies to help hold them together.

Another set of proposals is based on the possibility of a double metric tensor
Metric tensor
In the mathematical field of differential geometry, a metric tensor is a type of function defined on a manifold which takes as input a pair of tangent vectors v and w and produces a real number g in a way that generalizes many of the familiar properties of the dot product of vectors in Euclidean...

 for space-time. It has been argued that time reversed solutions in general relativity require such double metric for consistency, and that both Dark Matter and Dark Energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...

 can be understood in terms of time reversed solutions of general relativity.

Quantum Gravity



Quantum Gravity is an active wide-ranging theoretical physics field that encompasses many different competing theories, and even many different competing families of theories. It is also sometimes known as the Theory of Everything
Theory of everything
A theory of everything is a putative theory of theoretical physics that fully explains and links together all known physical phenomena, and predicts the outcome of any experiment that could be carried out in principle....

 or TOE. Basically, it is a class of theories that attempts to reconcile the two great not-yet-reconciled laws of physics, gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...

 with quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

, and obtain corrections to the current gravitational laws. Examples of quantum gravity theories are Superstring theory
Superstring theory
Superstring theory is an attempt to explain all of the particles and fundamental forces of nature in one theory by modelling them as vibrations of tiny supersymmetric strings...

, its successor M-Theory
M-theory
In theoretical physics, M-theory is an extension of string theory in which 11 dimensions are identified. Because the dimensionality exceeds that of superstring theories in 10 dimensions, proponents believe that the 11-dimensional theory unites all five string theories...

, as well as the competing Loop Quantum Gravity
Loop quantum gravity
Loop quantum gravity , also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the theories of quantum mechanics and general relativity...

.

In a sense, quantum gravity is a much more ambitious field of study than dark matter, since quantum gravity is an all-encompassing attempt to reconcile gravity with the other fundamental forces of nature, whereas dark matter is simply a classical physics solution for a classical gravity problem. It is hoped that once a testable quantum gravity theory emerges, that one of its side benefits will be to explain these various gravitational mysteries from first principles
First principles
In philosophy, a first principle is a basic, foundational proposition or assumption that cannot be deduced from any other proposition or assumption. In mathematics, first principles are referred to as axioms or postulates...

 rather than through empirical methods alone.

Some Superstring/M-Theory cosmologists propose that multi-dimensional forces from outside the visible universe have gravitational effects on the visible universe meaning that dark matter is not necessary for a unified theory of cosmology. M-Theory envisions that the universe is made up of more than the observable 3 spatial and 1 time dimensions, and that there are up to 11 dimensions altogether. The remaining dimensions are hidden from our full view and only show up at the quantum levels. However, if there are particles or energy that exist only within these alternate dimensions, then they might account for the gravitational effects currently attributed to dark matter.

Loop quantum gravity and its subset Loop quantum cosmology
Loop quantum cosmology
Loop quantum cosmology is a finite, symmetry-reduced model of loop quantum gravity that predicts a quantum bridge between contracting and expanding cosmological branches...

 envisions spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...

 itself as being made up of elementally small particles, or quanta
Quantum
In physics, a quantum is the minimum amount of any physical entity involved in an interaction. Behind this, one finds the fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization". This means that the magnitude can take on only certain discrete...

. This is quite different from how we usually envision empty space, as being simply empty, i.e. full of nothing: LQG and LQC says even empty space is actually made of something. Each particle of spacetime in various ways loops up (combines and twists) with adjacent particles of spacetime to create all of the matter and energy we see in the universe today. In this sense, if matter is just crumpled up spacetime, then even the empty untwisted space near a large body of matter would be put under more tension than empty untwisted space far away from matter; think of a long chain that you crumple up in the middle, the uncrumpled chainlinks near the crumpled up portion would still feel a large tension. This can be thought of as the same effect as dark matter. Chain links far away from the twists would feel little or no tension and would be in a state of relaxation, this can be analogous to dark energy.

In a 2004 study at the University of Mainz in Germany, it has been found that if one applies just a standard quantum mechanical approach to Newton's Gravitational constant
Gravitational constant
The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...

 at various scales within the astrophysical realm (i.e. scales from solar systems up to galaxies), it can be shown that the Gravitational constant is not so constant anymore and actually starts to grow. The implication of this is that if the Gravitational constant grows at different scales, then dark matter is not needed to explain galactic rotational curves.

Popular culture



Mentions of dark matter occur in some video games and other works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties. Such descriptions are often inconsistent with the properties of dark matter proposed in physics and cosmology.

See also


  • Cold dark matter
    Cold dark matter
    Cold dark matter is the improvement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation and whose constituent particles move slowly...

  • Hot dark matter
    Hot dark matter
    Hot dark matter is a hypothetical form of dark matter which consists of particles that travel with ultrarelativistic velocities. The best candidate for the identity of hot dark matter is the neutrino. Neutrinos have very small masses, and do not take part in two of the four fundamental forces, the...

  • Warm dark matter
    Warm dark matter
    Warm dark matter is a hypothesized form of dark matter that has properties intermediate between those of hot dark matter and cold dark matter, causing structure formation to occur bottom-up from above their free-streaming scale, and top-down below their free streaming scale. The most common WDM...

  • Light dark matter
    Light dark matter
    In astronomy and cosmology, light dark matter refers to dark matter Weakly interacting massive particle candidates with masses less than 1 GeV. These particles are heavier than warm dark matter and hot dark matter, but are lighter than the traditional forms of cold dark matter...

  • Mirror matter
    Mirror matter
    In physics, mirror matter, also called shadow matter or Alice matter, is a hypothetical counterpart to ordinary matter.Modern physics deals with three basic types of spatial symmetry: reflection, rotation and translation. The known elementary particles respect rotation and translation symmetry but...

  • Self-interacting dark matter
    Self-interacting dark matter
    In astrophysics self-interacting dark matter is a hypothetical form of dark matter consisting of particles with strong self-interactions. This type of dark matter was postulated to resolve a number of conflicts between observations and simulations on the galactic scale and smaller....

  • WIMPs
  • Chameleon particle
    Chameleon particle
    The "chameleon" is a postulated scalar particle with a non-linear self-interaction which gives the particle an effective mass that depends on its environment: the presence of other fields. It would have a small mass in much of intergalactic space, but a large mass in terrestrial experiments, making...

  • Multidark
    Multidark
    MULTImessenger Approach for DARK Matter Detection is a Spanish project with a stated goal of contributing to the identification and detection of the dark matter.-History:...

     (research program)
  • Dark matter halo
    Dark matter halo
    A dark matter halo is a hypothetical component of a galaxy, which extends beyond the edge of the visible galaxy and dominates the total mass. Since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas...

  • MACHOs
    Massive compact halo object
    Massive astrophysical compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter, which emits little or no radiation and drifts through interstellar...

  • Robust Associations of Massive Baryonic Objects (RAMBOs)
  • SIMP
    SIMP
    In astrophysics SIMP is an abbreviation of Strongly Interacting Massive Particle. SIMPs could form the inferred dark matter despite their strong interactions with ordinary matter....

  • Axion
    Axion
    The axion is a hypothetical elementary particle postulated by the Peccei-Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics...

  • Conformal gravity
  • Lambda-CDM model
    Lambda-CDM model
    ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy...

  • Galaxy formation and evolution
    Galaxy formation and evolution
    The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby...

  • Galaxy rotation curve
    Galaxy rotation curve
    The rotation curve of a galaxy can be represented by a graph that plots the orbital velocity of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis....

  • Modified Newtonian dynamics
    Modified Newtonian dynamics
    In physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...

  • Scalar field dark matter
    Scalar field dark matter
    In astrophysics and cosmology scalar field dark matter is a classical, minimally coupled, real scalar field postulated to account for the inferred dark matter.-Background:The universe may be accelerating, fueled perhaps by a cosmological constant or some...

  • Unparticle physics


Further reading


Invited talk at the 36th COSPAR Scientific Assembly, Beijing, China, 16–23 July 2006

External links