Transcript: If black holes are totally dark, how can they possibly be detected? A black hole that was isolated in space would indeed by very difficult to detect, but if it’s in a binary system it is possible. Astronomers look for binary systems where one member of the pair is massive enough to have left a core three times the mass of the Sun or larger. In a binary system mass is pulled from the companion onto the black hole and accelerated. The gas forms a disk around the black hole called an accretion disk, and the hot accretion disk emits ultraviolet and especially soft x-ray emission. So to find black hole candidates astronomers start with x-ray surveys and look for intense x-ray emitting stars. They then look for evidence of a binary system and especially a dark companion with a mass of three times the mass of the Sun or greater. About a dozen excellent systems are known where it’s very likely that a dark companion black hole exists. Are black holes perfectly proven? No, but if astronomers were asked for the likelihood that black holes exist most would say its 90 or 95 percent proven.