A black hole is a region of spacetime where the gravitational pull is so strong that nothing, not even light, can escape. The boundary of this region is called the event horizon. Black holes form when very massive stars collapse at the end of their life cycle. Supermassive black holes millions of times the mass of our sun may form at the centers of galaxies by absorbing other stars and merging with other black holes. While black holes themselves are invisible, their presence can be inferred through their interaction with surrounding matter like accretion disks and the orbits of nearby stars.