Black holes are objects for which the gravitational attraction is so strong that nothing, not even light, can escape from it. They exist in the universe in large numbers.

Albert Einstein's theory of general relativity explains the properties of black holes.^* The material inside a black hole is concentrated into a singularity: a single point of infinitely high density where space and time are infinitely distorted. Distant objects can escape from a black hole's gravitational pull, but objects inside the so-called event horizon inevitably fall toward the center (such objects would have to move faster than light to escape, which is impossible according to the laws of physics). The size of the event horizon and the distortions of the space and time surrounding it are determined by the mass and spin (rate of rotation) of the black hole. Space and time distortions cause unusual effects; for...