The basic argument is that if we measure velocities in some region, then there has to be enough mass there for gravity to stop all the objects from flying apart. When such velocity measurements are done on large scales, it turns out that the amount of inferred mass is much more than can be explained by the luminous mass. Hence we infer that there is non-luminous matter in the Universe, i.e. there is dark matter.

Dark matter has important consequences for the evolution of the Universe. According to standard cosmological theory, the Universe must conform to one of three possible types: open, flat, or closed. A parameter known as the "mass density" - that is, how much matter per unit volume is contained in the Universe - determines

As leading possible candidates for baryonic dark matter, there are black holes (large and small), brown dwarfs (stars too cold and faint to radiate), sun-size MACHOs, cold gas, dark galaxies and dark clusters, to name only a few. The range of particles that could constitute non-baryonic dark matter is limited only slightly by theorists' imaginations. The particles include photinos, neutrinos, gravitinos, axions, and magnetic monopoles, among many others. Of these, researchers have detected only neutrinos -- and whether neutrinos have any mass remains unknown. Experiments are under way to detect other exotic particles. If they exist, and if one has a mass in the correct range, then that particle might pervade the universe and constitute dark matter.

Dark matter candidates are usually split into two broad categories, with the second category being further sub-divided: baryonic and bon-baryonic. Then, under non-baryonic, hot dark matter (HDM) and cold dark matter (CDM) are its types. Depending on their respective masses and speeds, CDM candidates have relatively large mass and travel at slow speeds (hence "cold"), while HDM candidates include minute-mass, rapidly moving (hence

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