It's there, you just can't see it: Astronomers have found evidence that there appears to be more mass present in the universe than can be accounted for by luminous material. It has been estimated that the stars, gas, and dust we can observe account for only about 1%-10% of the universe's total mass. Candidates for what constitute the dark matter include dim stars, black holes, white dwarf stars, gas, and dimly lit galaxies (i.e., large collections of particles; collectively called MACHOs, or massive astronomical compact halo objects), as well as neutrinos or other exotic particles (i.e., individual particles; collectively called WIMPs, or weakly-interacting massive particles).