Einstein's general theory of relativity describes gravity as a curvature of spacetime 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.
Do black holes actually exist? Most physicists believe they do, basing their views on a growing body of observations. In fact, present theories of how the cosmos began rest in part on Einstein's work and predict the existence of both singularities and the black holes that contain them. Yet Einstein himself vigorously denied their reality, believing, as did most of his contemporaries, that black holes were a mere mathematical curiosity. He died in 1955, before the term "black hole" was coined or understood and observational evidence for black holes began to mount.
Why Study Black Holes?
Here are some good reasons:
1. Human curiosity: they are among the most bizzare objects thought to exist in the universe.
2. They should be strong sources of gravitational waves.
3. As such, black holes should reveal much about gravity, a fundamental force in the cosmos.
4. Confirmation that they exist will strengthen confidence in current models of cosmic evolution, from the Big Bang to the present universe.