MACHOs, or Massively Compact Halo Object, is the name given to astronomical objects — such as dwarf stars or lone planets stripped from their suns — that are too dim to easily detect. MACHOs were considered a candidate for dark matter, the mysterious substance that makes its presence felt only through gravity, but even the faintest red or white dwarf emits some detectable energy. MACHOs have never been found in sufficient quantity to account for all the unseen matter necessary to explain observed galaxy growth and behavior.
Currently the largest component of dark matter is generally considered to be some form of WIMP, or Weakly Interacting Massive Particle, even though WIMPs have never been detected. The Cryogenic Dark Matter Survey is one attempt to find WIMPs.
From SLAC Today; used with permission.