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Dark matter is only a theory.
Dark matter was first hypothesized to exist by the Swiss astrophysicist Fritz Zwicky. In 1933 Zwicky estimated the total amount of mass in a cluster of galaxies, the Coma Cluster, based on the motions of the Galaxies near the edge of the cluster. When he compared this mass estimate to one based on the number of Galaxies and total brightness of the cluster, he found that there was about 400 times more mass than expected. 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 other form of matter existent in the cluster which we have not detected, which provides enough of the mass and gravity to hold the cluster together.
In cosmology, Dark Matter consists of matter particles that cannot be detected by their emitted radiation but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. Estimates of the amount of matter in the galaxies, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. In addition, the existence of Dark Matter resolves a number of inconsistencies in the Big Bang theory, and is crucial for structure formation.
Much of the mass of the Universe is believed to exist in the "dark sector". Determining the nature of this missing mass is one of the most important problems in modern cosmology. About 25% of the Universe is thought to be composed of dark matter, and 70% is thought to consist of dark energy, an even stranger component distributed diffusely in space that likely cannot be thought of as ordinary particles.
At present, the density of ordinary baryons and radiation in the Universe is estimated to be about one Hydrogen atom per cubic meter of space. However, Dark Matter and Dark Energy are together said to account for 96% of all matter in the universe. This means that only about 4% of all matter can be directly observed.
Much of the evidence for Dark Matter comes from the study of the motions of galaxies. Many of these appear to be fairly uniform, so by the virial theorem the total kinetic energy should be half the total gravitational binding energy of the galaxies. Experimentally, however, it is found to be much greater: in particular, 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 spherical Halo of Dark Matter with the visible matter concentrated in a disc at the center. Low surface brightness dwarf Galaxies 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 impair observations of the rotation curve of outlying stars.
Recently, Astronomers from Cardiff University claim to have discovered a Galaxy made almost entirely of dark matter, 50 million light years away in the Virgo Cluster, which was named VIRGOHI21. 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 as much Dark Matter as Hydrogen and has a total mass of about 1/10th of 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 and structure formation have suggested that such dark Galaxies should be very common in the universe, but none have previously been detected. If the existence of this dark Galaxy is confirmed, it provides strong evidence for the theory of Galaxy formation and pose problems for alternative explanations of dark matter.
Dark matter is believed to affect Galaxy clusters as well. The Galaxy cluster Abell 2029 is composed of thousands of Galaxies enveloped in a cloud of hot gas, and an amount of Dark Matter equivalent to more than a hundred trillion 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.
A significant amount of non-baryonic, cold matter is necessary to explain the large-scale structure of the universe. Observations suggest that structure formation in the Universe proceeds hierarchically, with the smallest structures, such as stars, forming first, and followed by Galaxies and then clusters of galaxies. In the universe, it is thought that the first structures that form are quasars, which are primeval stars. This, bottom up model of structure formation requires something like cold Dark Matter to succeed. 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.
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 and 2dF Galaxy Redshift Survey, as well as observations of the Lyman-alpha forest. These studies have been crucial in constructing the Lambda-CDM model which measures the cosmological parameters, including the fraction of the Universe made up of baryons and dark matter.
Another important tool for future Dark Matter observations is gravitational lensing, in particular a technique called weak lensing that allows astrophysicists to characterize the distribution of Dark Matter by statistical means.
Data from Galaxy rotation curves indicate that nearly 90% of the mass of a Galaxy cannot be seen. It can only be detected by its gravitational effect. Several categories of Dark Matter have been postulated.
Hot dark matter.
Cold dark matter.
Baryonic dark matter.
Hot Dark Matter consists of particles that travel with relativistic velocities. One kind of hot Dark Matter is known, the neutrino. Neutrinos have a very small mass, do not interact via either the electromagnetic or the strong nuclear force and so are incredibly difficult to detect. This is what makes them appealing as dark matter. However, bounds on neutrinos indicate that ordinary neutrinos make only a small contribution to the density dark matter.
Hot Dark Matter cannot explain how individual Galaxies formed from the Big Bang. The microwave background radiation as measured by the COBE and WMAP satellites, while incredibly smooth, indicates that matter has clumped on very small scales. Fast moving particles, however, cannot clump together on such small scales and, in fact, suppress the clumping of other matter. Hot dark matter, while it certainly exists in our Universe in the form of neutrinos, is therefore only part of the story.
To explain structure in the Universe it is necessary to invoke cold (non-relativistic) dark matter. Large masses, like galaxy-sized black holes can be ruled out on the basis of gravitational lensing data. Possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements; such objects are known as massive compact Halo objects, or "MACHOs". However, studies of Big Bang nucleosynthesis have convinced most scientists that baryonic matter such as MACHOs cannot be more than a small fraction of the total dark matter.
At present, the most common view is that Dark Matter is made of one or more elementary particles other than the usual electrons, protons, and neutrons. Currently, the most commonly considered particles are neutrinos, axions, SIMPs (Strongly Interacting Massive Particles), and WIMPs (Weakly Interacting Massive Particles). None of these are part of the standard model of particle physics. Instead, particles in this last category are frequently suggested by theorists proposing supersymmetric extensions of the standard model of particle physics. In such theories, the WIMP involved is usually the neutralino. Another candidate is so-called sterile neutrinos. Sterile neutrinos can be added to the standard model to explain the small neutrino mass. These sterile neutrinos are expected to be heavier than the ordinary neutrinos, and are a candidate for dark matter.
Since it cannot be directly detected via optical means, many aspects of Dark Matter remain speculative. The DAMA/NaI experiment has claimed to directly detect Dark Matter passing through the Earth, though most scientists remain sceptical since negative results of other experiments are (almost) incompatible with the DAMA results if Dark Matter consists of neutralinos.
An alternative to Dark Matter is to suppose that the inconsistencies are due to an incomplete understanding of gravitation. To explain the observations, the gravitational force has to become stronger than the Newtonian approximation at great distance. For instance, this can be done by assuming a negative value of the cosmological constant (the value of which is believed to be positive based on recent observations) or by assuming Modified Newtonian Dynamics (MOND), which corrects Newton's laws at small acceleration. However, constructing a relativistic MOND theory has been troublesome, and it is not clear how the theory can be reconciled with gravitational lensing measurements of the deflection of light around galaxies. The leading relativistic MOND theory, proposed by Milgrom's colleague Professor Bekenstein in 2004 is called "TeVeS" for Tensor-Vector-Scalar and solves many of the problems of earlier attempts.
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