Black hole artwork. A black hole (centre) is a region in space where the pull of gravity exceeds the velocity of light. This black hole is surrounded by an accretion disc, matter spiralling into the black hole. Once the matter passes the point at which the gravitational field exceeds the speed of light (the event horizon), it can no longer be seen by an external observer. This region thus appears black. Black holes can form when a large star collapses under its own gravity at the end of its life. Most galaxies are thought to harbour massive black holes at their centres.