Massive compact halo object

Massive compact halo objects, or MACHOs, are a type of astronomical body proposed as one possible explanation for the presence of dark matter in galactic halos. A MACHO is a small chunk of normal baryonic matter, far smaller than a star, which drifts through interstellar space unassociated with any solar system. Since MACHOs would not emit any light of their own, they would be very hard to detect.

Recent work has suggested that MACHOs are not likely to account for the large amounts of dark matter now known to be present in the universe; the Big Bang as it is currently understood simply couldn't produce enough baryons. However, some MACHOs may still exist (at the upper end of their size scale they would be brown dwarfs) and there are some attempts being made to detect them by the gravitational lensing they would cause in the light of stars as they pass in front of them.