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# Dark matter

In cosmology, dark matter consists of elementary 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 universe, 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.

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.

The question of the existence of dark matter may seem irrelevant to our existence here on Earth. However, whether or not dark matter really exists could determine the ultimate fate of the present universe. We know the universe is now expanding because of the red shift that light from distant heavenly bodies exhibits. The amount of ordinary matter seen in the universe is not enough for gravity to stop this expansion, and so the expansion would continue forever in the absence of dark matter. In principle, enough dark matter in the universe could cause the universe's expansion to stop or even reverse (leading to an eventual Big Crunch).

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## Evidence for dark matter

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 to keep such fast-moving galaxies bound, 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.

From there the search for this source of the sufficient gravity has commenced. At present, the density of ordinary baryons and radiation in the universe is estimated to be about one hydrogen atom per cubic meter of empty space. However, dark matter and dark energy are said to form 90–95% of all matter in the universe. This means that only 5–10% of all matter is directly observed.

Cosmologists (astrophysicists who study the history, origin, and future of the universe) believe there are two classes of dark matter: baryonic (the name given to all "normal matter" composed of baryons: protons, neutrons) dark matter, called massive compact halo objects (MACHOs), and the mysterious "shadow matter" composed of hypothetical non-baryonic subatomic particles, specific candidates of which include axions, WIMPs, simps, neutrinos, mirror matter, and the big-particle hypothesis. (The acronym MACHO was chosen specifically to contrast with the theory of WIMPs.)

Much of the evidence for dark matter comes from the study of large-scale structure such as galaxies and galaxy clusters. Many of these appear to be roughly static and 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, often off by an order of magnitude or so, and 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 dark matter – for instance the rotation curves in and indeed the very existence of our galaxy's disc are most easily explained if the galaxy contains an extended dark matter halo. With gravitational theory and new computer analyses, astronomers have now been able to work out where the dark matter appears to be. The results are just what you would expect if dark matter and galaxies are similarly clustered. Knowing where the dark matter is also reveals how much of it exists: about seven times as much as ordinary matter (thought to be one quarter of what is necessary to slow down the universe's expansion to a halt). Another important tool for observing dark matter is gravitational lensing, in particular a technique called weak lensing that allows astrophysicists to characterize the distribution of dark matter by statistical means.

The galaxy cluster Abell 2029 is composed of thousands of galaxies enveloped in a gigantic 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. More info is available here: http://chandra.harvard.edu/photo/2003/abell2029/

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 (Wikinews , New Scientist). Contrary to some other alleged dark matter objects, 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 will help to vindicate the theory of galaxy formation and pose problems for alternative explanations of 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.

## Alternative explanations

An alternative to dark matter is to suppose that gravitational forces 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.

Another approach, proposed by Finzi (1963) and again by Sanders (1984), is to replace the gravitational potential energy with the expression

$U=\frac{GM(1-Be^{-r/\rho})}{(1-B)r}$

where B and ρ are adjustable parameters. However, such approaches run into difficulties explaining the different behavior of different galaxies and clusters, whereas one can easily describe such differences by assuming different quantities of dark matter.

For a deeper discussion of this subject, see Modified Newtonian dynamics.

Another proposed explanation of the mystery is Nonsymmetric Gravitational Theory.

## Composition

Data from galaxy rotation curves indicate that almost 90% of the mass of a galaxy cannot be seen. It can only be detected by its gravitational effect. There are several types of dark matter postulated to exist.

Hot dark matter consists of particles that travel with relativistic velocities. One kind of hot dark matter is known, the neutrino. Neutrinos have nearly negligible 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, supress 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 Weakly Interacting Massive Particles (WIMPs). The last category is 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.

## Related topics

• Dark Matter is sometimes portrayed as a tangible item with particularly powerful properties in RPGs, notably Wizards of the Coast's 'Dark Matter' conspiracy roleplaying game.