A black hole is an object, usually a collapsed star, that is so dense that within a certain radius its escape velocity exceeds the speed of light. Therefore, nothing within that radius - not even light - can escape. Its strong gravity absorbs all light and matter within that radius, leaving nothing to escape.
Black holes are a popular theme for scientific speculations, such as white holes and wormholes.
Because the general relativity equations describing black holes are symmetric with respect to time, one can take a negative square root instead of a positive to yield a new equation describing a hypothetical object which expels, rather than attracts, matter and energy. Because this object would be the exact opposite of a black hole, it is called a white hole.
Essentially, a white hole would be a time-reversed black hole - matter and energy could leave its event horizon, but none could enter. Since matter and energy would be apparently coming out of nowhere, white holes would appear to violate the law of conservation of mass-energy. To avoid this problem, some scientists have proposed that matter falling into black holes goes through a wormhole to emerge at a white hole. While such wormholes are allowed by general relativity, they would be extremely unstable. Some scientists thought that pulsars are white holes, but they have since proven to be otherwise. No white holes or wormhole has ever been identified.