For as long as the Earth has been around, humans have looked up in awe at the night sky and wondered about the intricate pieces that make up what is our universe. For most of Earth's history, the universe only encompassed what was visible to the naked eye. In 1609, however, everything changed when telescopes were invented and the realm of the universe opened to unveil new and mysterious aspects never before seen. Interestingly enough, though technology has grown to gigantic proportions, there is still a lot we don't know about the universe, and black holes, white holes, and wormholes are just a few of the aspects we still have much to learn about. At this point in time, astronomists have yet to actually see a black hole. So how do they know that black holes do in fact exist? And how can they deduce that related beings like white holes and wormholes do (or do not) exist? Through careful deduction and intensive examination, scientists like Einstein, Stephen Hawking, and many others have been able to give insight into the deep, enigmatic universe and exactly what these mysterious beings are all about.
Einstein's general theory of relativity describes gravity as a curvature of space time caused by the presence of matter. If the curvature is fairly weak, Newton's laws of gravity can explain most of what is observed, for example, the regular motions of the planets. Very massive or dense objects generate much stronger gravity. The most compact objects imaginable are predicted by General Relativity to have such strong gravity that nothing, not even light, can escape their grip. Scientists today call such an object a black hole. Why black? Though the history of the term is interesting, the main reason is that no light can escape from inside a black hole: it has, in effect, disappeared from the visible universe.
By definition a black hole is a region where matter collapses to infinite density and where, as a result, the curvature of spacetime is extreme.