The warped space-time remaining after the gravity of a massive body has caused it to shrink down to a point. It is a region of empty space with a point-like singularity at the centre and an event horizon at the outer edge. It is so dense that no normal matter or radiation can escape its gravitational field, so that nothing - not even light - can ever leave (hence its blackness). It is thought that most galaxies have a supermassive black hole at their heart.