A black hole is not a hole in the conventional sense of the word. It is the region of space around a massive collapsed star. A black hole consists of an astronomical body with so high a gravitational field that it curves space around it. As well as attracting all nearby objects to itself, it attracts the sheer fabric of space-time around its massive body. It causes "gravitational self-closure" i.e. a region is formed around the black hole that neither particles nor photons (light) can escape from. The body is surrounded by a spherical boundary, called a horizon or event horizon, through which light can enter but not escape and therefore appears totally black.

Karl Schwarzschild developed the concept of a black hole in 1916 following Einstein's theory of general relativity. Einstein's theory proposed that gravitation from a black hole severely alters space-time. As the horizon is approached from the outside, time slows down relative to that of distant observers, stopping completely on the horizon. The radius of the horizon depends on the mass of the body; the denser it is, the larger the radius.

It is believed that the most promising candidates for black holes are massive stars that explode as supernovae. A supernova is a star that explodes after its internal nuclear fuel is exhausted. The explosion can leave behind a core in excess of three solar masses. A core of this mass must undergo complete gravitational collapse because it is above the stable limit for white dwarfs (low mass stars in the last stage of evolution) and neutron stars (stable collapsed stars). The star continues to shrink and become denser until its escape velocity (the velocity required for an object to break free of a body's gravitational field) equals the speed of light. Once a body has contracted within a defined radius, it would theoretically collapse to a singularity i.e. a dimensionless object of infinite density.

It has been theorised that supermassive black holes probably lie at the centre of some galaxies, and indeed our own Milky Way. In 1994, the Hubble Space Telescope photographed the first convincing evidence towards the existence of black holes. It is impossible to see a black hole and the only method of detection is to study the effects upon some nearby object that can be identified.