Some have suggested that dark matter may be rather plain, with sizes like baseballs or schoolbuses, or even mountains, because such objects would be extremely difficult to detect. They don't absorb, emit, or reflect all that much light. We also wouldn't require so many that we could just wait around until they hit the earth. Such objects would be nearly impossible to detect over the background of interplanetary clutter that regularly encounters the earth. However, if the objects are as massive as planets, we call them "MACHO's" for MAssive Compact Halo Objects, and an extensive search for such objects is underway, using their subtle gravitational lensing properties as a probe.
Others believe the dark matter is composed of some supersymmetric particle which is a relic from the early universe. These particles react only very weakly to normal matter. Already, we know that neutrinos could could pass through trillions of miles of lead or concrete without interacting, so still more weakly interacting particles might exist but completely elude detection. However, unlike neutrinos, these particles must have significant mass. The zoo of possible particles proposed by theorists is simplified to "WIMP's" or Weakly Interacting Massive Particles. Elaborate searches for this dark matter candidate are underway also, although the lack of the Superconducting Supercollider damaged the cause somewhat.
Exotic particles or baseballs, WIMP's or MACHO's, the dark matter composes 90% of the mass of the galaxy, and probably 97% of the mass of the Universe. Just what the Universe is made of remains a complete mystery, with only faint clues about its nature.
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