A number of theories that propose to explain what happened in the very early universe (the first small fraction of a second) and link elementary particle physics and cosmology predict the existence of exotic particles that have yet to be discovered. IF these particles exist, they could contribute significantly to the dark matter in the universe.
One category of exotic particles includes the WIMPs: Weakly Interacting Massive Particles, including the photino, the axion, and the squark. Because these particles are relatively massive (more massive than the neutrino, for example, though axions have a mass still one million times smaller than the electron), they would travel at relatively low velocities and therefore could be considered "cold". We refer to these particles as "cold dark matter" (as opposed to "hot dark matter" like neutrinos). Each of these particles could be very common and very hard to observe! Their purpose is to allow the unification of theories that govern physics in the first 10-35 second. There is no evidence that such particles exist --- but maybe they do!
``The Dark Side of the Universe'' Trefil, James. 1988 (Doubleday, Anchor Books, New York).
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