At least the names for the two main possible culprits are entertaining: they are said to be either WIMPs (for Weakly Interacting Massive Particles, which is to say specks of invisible matter left over from the Big Bang) or MACHOs (for MAssive Compact Halo Objects—really just another name for black holes, brown dwarfs, and other very dim stars).
Particle physicists have tended to favor the particle explanation of WIMPs, astrophysicists the stellar explanation of MACHOs. For a time MACHOs had the upper hand, but not nearly enough of them were found, so sentiment swung back toward WIMPs but with the problem that no WIMP has ever been found. Because they are weakly interacting, they are (assuming they even exist) very hard to detect. Cosmic rays would cause too much interference. So scientists must go deep underground.
One kilometer underground cosmic bombardments would be one millionth what they would be on the surface. But even when all these are added in, "two-thirds of the universe is still missing from the balance sheet," as one commentator has put it. For the moment we might very well call them DUNNOS (for Dark Unknown Nonreflective Nondetectable Objects Somewhere).
Recent evidence suggests that not only are the galaxies of the universe racing away from us, but that they are doing so at a rate that is accelerating.