Dark matter is a hypothetical kind of matter that cannot be seen with telescopes, but accounts for most of the universe's mass. The existence and properties of dark matter are inferred from its gravitational effects on visible matter, on radiation, and on the universe's large-scale structure. Dark matter has not been directly detected, leaving it an astrophysical mystery.

Dark•Matter is a science fiction/conspiracy theory campaign setting that was originally published in 1999 by Wizards of the Coast as the second campaign setting for the Alternity role-playing game. It was written by Wolfgang Baur and Monte Cook. It was later converted to d20 Modern rules and published as a stand-alone book in 2006.