Definition : material which may exist in the universe but be 'invisible' in the sense that not only does it no shine with its own light nor yet reflect light, but further it reacts rarely or weakly with the visible matter that it is difficult to detect in any way. Its main influence would be by gravitational attraction.

Theoretical considerations

        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 based on the amount of visible matter that appears to be in these galaxies. Many astronomers believe there is enough evidence to conclude that, based on its rate of rotation, up to 90 percent of the matter in a typical galaxy is invisible.

        The second theoretical consideration is based on the presence of clusters of galaxies. Many galaxies in the universe are grouped together in clusters. Some astronomers argue that if the clustered galaxies are bound together by gravity, and that the clusters formed billions of years ago-then it follows that more than 90 percent 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.

        The third theoretical consideration that suggests that dark matter exists is based on the inflationary big bang model. The inflationary big bang model asserts that 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 for the expansion of the universe is close to one. In order for this constant to be near one, the total mass of the universe must be more than 100 times the amount of visible mass that appears to be in the universe. This calculation indicates that more than 99 percent of the mass of the universe must arise from dark matter.

Types of Dark Matter

        Theorized kinds of dark matter include cold dark matter, with slowly moving (cold) massive particles. No such particles have yet been detected, though astronomers have given them names like Weakly Interacting Massive Particles (WIMPs). Other cold dark matter could be nonradiating stars or planets, which are known as MACHOs (Massive Compact Halo Objects).

        An alternative model includes hot dark matter, where hot implies that the particles are moving very fast. The fundamental particles known as neutrinos are the prime example of hot dark matter.

        If the inflationary version of big bang theory is correct, then the amount of dark matter that exists is just enough to bring the universe to the boundary between open and closed. Scientists develop theoretical models to show how the universe's structures, such as clusters of galaxies, have formed. Their models invoke hot dark matter, cold dark matter, or a mixture of the two. This unseen matter would have provided the gravitational force needed to hold large structures such as clusters of galaxies together. The theories continue to match the observations, though there is no consensus on the type or types of dark matter that must be included.

Candidates for Dark Matter

        There are several possible candidates for the material that makes up dark matter. These include neutrinos with mass, undetected brown dwarfs, white dwarf stars, black holes, and exotic subatomic particles whose properties preclude detection by observing electromagnetic radiation.

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