If dark matter does exist, it has vastly more mass than the "visible" part of the universe [1]. Only about 4% of the total mass in the universe (as inferred from gravitational effects) can be seen directly. 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 and particle physics.

Hypothesizing 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) Professor Peebles and Professor Ostriker, both of Princeton University investigated dark matter.

The first to hypothesize dark matter was Swiss astrophysicist Fritz Zwicky, of the California Institute of Technology (Caltech) in 1933. He applied the virial theorem to the Coma cluster of galaxies and obtained evidence of unseen mass. 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.

Unsolved problems in physics: What is dark matter? How is it generated? Is it related to supersymmetry?

Professor T. Sumner of Imperial College London hypothesized a possible particle, the neutralino, which is a candidate for Cold dark matter but so far it is undetected. Sumner was looking for something massive which fails to interact with normal matter. If dark matter exists throughout the Milky Way Galaxy it must pass through the Earth but fails to interact with the Earth or with us. Professor Sumner and his team have a detector at the bottom of Europe's deepest mine in Cleveland, England. There he hopes cosmic rays and surface particles will not distort the result. Currently nothing has been found. [2]

Evidence for dark matter

At present, the density of ordinary baryons and radiation in the universe is estimated to be equivalent to 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. Some hard-to-detect baryonic matter (see baryonic dark matter) makes a contribution to dark matter, but constitutes only a small portion [3] [4].

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.

Recent research reported in January 2006 from the University of Massachusetts, Amherst would explain the previously mysterious warp in the disk of the Milky Way by the interaction of the Large and Small Magellanic Clouds and the predicted 20 fold increase in mass of the Milky Way taking into account dark matter.

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, the total kinetic energy 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.

In 1974 Vera Rubin, now of Carnegie Institution of Washington, found that most stars in spiral galaxies orbit at roughly the same speed. This is known as the galaxy rotation problem. This problem suggests that either Newtonian gravity does not apply universally or that there is dark matter. [5]

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 more dark matter than hydrogen and has a total mass of about 1/10th 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 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. More info is available here: [6] .

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 supermassive black holes. 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 a number of lines of 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, indicate that 85-90% of the mass in the universe does not interact with the electromagnetic force. This "dark matter" is evident through its gravitational effect. Several categories of dark matter have been postulated.

• Hot dark matter
• Warm 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 are therefore 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.

In research due to be fully published in spring 2006, researchers from the University of Cambridge Institute of Astronomy claim to have calculated that dark matter only comes in clumps larger than about 1,000 light-years across, implying an average speed of dark matter particles of 9km/s, corresponding to a relatively warm 10,000 degrees Celsius [7].

Alternative explanations

An alternative to dark matter is to suppose that the inconsistencies are due to an incomplete understanding of gravitation. One approach is to attempt to reconcile gravitation with quantum mechanics and to explain mass and its creation within gravitation, as in some scalar-tensor theories, which couple scalar fields like the Higgs field 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 to explain 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

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 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 [8].

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 [9]. 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 [10]. 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". Cooperstock and Tieu have since published an article in which they argued that the thin disk was an artefact of approximations made by their model. However, in a comment on that article, D. Vogt and P. S. Letelier have disputed this. They show that the removal of the thin disk generates two other singular mass surface layers.