macho

n. [ acronym from Massive Astrophysical Compact Halo Object. ] A form of dark matter in distant outer space unobservable except by its gravitational effect, and believed to be at least part of the “missing matter” which is unobservable as ordinary stars, but helps keep galaxies from flying apart; MACHOS are massive but compact objects such as neutron stars or brown dwarfs, which can be detected (with difficulty) by their effect in bending light from distant light sources, such as other galaxies. This can occur if by rare chance a MACHO passes in front of a more distant visible object, and the light from that object becomes temporarily amplified by the MACHO acting as a gravitational lens. Some MACHOs have been discovered in the halo of dark matter that surrounds our milky way. It is, however, questionable whether the amount of such matter is sufficient to explain the ability of galaxies to stay together in spite of rotation rates that would cause them to fly apart if the only matter present was that observable as visible stars. [ PJC ]