# Dark matter

In cosmology, dark matter refers to hypothetical matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. The dark matter hypothesis aims to explain several anomalous astronomical observations, such as anomalies in the rotational speed of galaxies (the galaxy rotation problem). Estimates of the amount of matter present in galaxies, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. The existence of dark matter would also resolve a number of inconsistencies in the Big Bang theory, and is crucial for structure formation.

If dark matter does exist, it vastly outmasses the "visible" part of the universe [1]. Only about 4% of the total mass in the universe (as inferred from gravitational effects) can be accounted for. About 23% is thought to be composed of dark matter. The remaining 73% is thought to consist of dark energy, an even stranger component, distributed diffusely in space, that probably cannot be thought of as ordinary particles. Determining the nature of this missing mass is one of the most important problems in modern cosmology.

## Evidence for dark matter

In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space. (Ref. See notes)

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.

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.

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.

### Galactic rotation

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 (Wikinews, New Scientist). 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 poses 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. More info is available here: http://chandra.harvard.edu/photo/2003/abell2029/.

### Structure formation

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.

## Composition

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 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 of 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 most dark matter is made of one or more elementary particles other than the usual electrons, protons, neutrons, and ordinary neutrinos. Currently, the most commonly considered particles are axions, sterile neutrinos, SIMPs (Strongly Interacting Massive Particles), and WIMPs (Weakly Interacting Massive Particles) (which include neutralinos). 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.

## Alternative explanations

An alternative to dark matter is to suppose that the inconsistencies are due to an incomplete understanding of gravitation. One task could be given through the need of conciling gravitation with quantum mechanics and to explain mass and its creation (Higgs) within gravitation, as in some scalar-tensor theories, which couple scalar fields like the Higgs one to the curvature given through the Riemann tensor or its traces. In many of such theories, the scalar field equals the inflaton field, which is needed in some theories for explaining the inflation of the universe after the Big Bang, as the dominating factor of the quintessence or Dark Energy.

To explain the observations, the gravitational force has to become stronger than the Newtonian approximation at great distances or in weak fields. 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.

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

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

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.

Two other theories which propose modifications to general relativity have recently been proposed. M. Reuter and H. Weyer have proposed that Newton's constant grows at large scales due to quantum effects [2]. Another proposal by Cooperstock and Tieu suggested that the galaxy rotation problem could be explained with the results of general relativity, amplified by non-linear effects so that the behavior of the galaxy as a whole becomes non-Newtonian [3]. A problem in this model was found when it was shown that this model gives rise to a "thin, singular disk" of 2-dimensional matter in the galactic plane [4]. To save the model the physical nature of this disk must be addressed and be shown to be consistent with observations. In a recent article it is shown that Cooperstock's and Tieu's model implies that the thin disk must be made out of "exotic matter, either cosmic strings or struts with negative energy density".

## Dark matter in popular culture

Mentions of dark matter are seen occasionally in video games and other media.

• In several games produced by Squaresoft, including Chrono Trigger, Final Fantasy V, Final Fantasy VIII, and Final Fantasy X, dark matter exists as a powerful magical element, enabling certain kinds of major attacks.
• Dark Matter plays a central role in the His Dark Materials - Trilogy by British author Philip Pullman.
• In many of the Kirby videogames, Dark Matter is an evil entity from space that posesses characters, such as King Dedede, to do its bidding.
• In Golden Sun: The Lost Age, dark matter is used in forging very powerful but cursed equippable items.
• In the popular comedy cartoon series Futurama, dark matter was a very heavy piece of fuel for the show's starships, excreted by Leela's pet Nibbler.
• Dark matter is briefly mentioned in the description of a debris field in the Kepler and Galileo systems of Freelancer, a space combat game. Despite the use of dark matter in context in this last case, there is little similarity between the dark matter of Freelancer and anything we know about dark matter in real life.
• The webcomic Schlock Mercenary involves several battles with dark matter entities, who have been plotting to destroy the galaxy for several hundred thousand years.
• Dark Matter is the title of a science fiction novel by Garfield Reeves-Stevens involving mystery, horror, and physics.
• In Final Fantasy IX, Dark Matter was used to summon Odin.
• In the Alternity campaign setting Star*Drive, dark matter decay is used to fuel most modern starships as part of a "mass reactor." This reactor, in conjunction with a stardrive, makes FTL travel possible.
• In one episode of Star Trek: Voyager, the space craft encounters and destroys a Dark Matter asteroid.
• In the animated television series Exosquad, dark matter was a material found naturally on the tenth planet of the Solar system, Chaos. The Pirate Clans and the Exofleet used it to cloak their spaceships.