Dark Matter is classified as nonluminous material that cannot be directly detected by observing any form of electromagnetic radiation, but whose existence, distributed throughout the universe, is suggested by certain theoretical considerations. Determining whether dark matter exists, and in what quantity, are some of the most challenging problems in modern astrophysics.
Three principal theoretical considerations suggest that dark matter exists. The first is based on the rotation rate of galaxies. Galaxies near the Milky Way appear to be rotating faster than would be expected from the amount of visible matter that appears to be in these galaxies. Many astronomers believe there is enough evidence to conclude that up to 90 per cent of the matter in a typical galaxy is invisible.
The second theoretical consideration is based on the existence of clusters of galaxies. Many galaxies in the universe are grouped into such clusters. Some astronomers argue that if some reasonable assumptions are accepted-specifically, that the clustered galaxies are bound together by gravity, and that the clusters formed billions of years ago-then it follows that more than 90 per cent of the matter in a given cluster is made up of dark matter; otherwise clusters would lack enough mass to keep them together, and the galaxies would have moved apart by now. In 1998 two sets of observations changed the premises of this scenario; X-Ray observations of gas in intergalactic clouds using the ROSAT satellite showed that galaxies had formed individually before they began to group together in clusters and superclusters; and studies of very faint galaxies using the Hubble Space Telescope hinted at an inverse relationship between dark and normal matter, with the smallest, faintest galaxies having motions that indicated the presence of the greatest amount of dark matter.
The third theoretical consideration that suggests that dark matter exists is based on the inflationary big bang model. Of the three types of consideration suggesting the existence of dark matter, this is the most controversial. According to the idea of cosmic inflation, the universe went through a period of extremely rapid expansion when very young. However, if the inflationary big bang model is correct, then the cosmological constant describing the expansion of the universe is close to 1. In order for this constant to be near 1, the total mass of the universe must be more than 100 times the amount of visible mass that appears to exist.
There are several possible candidates for the material that makes up dark matter. 

These include: 

	neutrinos with mass
	undetected brown dwarfs (objects, resembling stars, that are smaller and much fainter than the Sun and are not powered by nuclear reactions)
	black holes; and exotic subatomic particles whose properties preclude detection by observing electromagnetic radiation. from: "Dark Matter," Microsoft® Encarta® Encyclopedia 2000. © 1993-1999 Microsoft Corporation. All rights reserved.