Albert Einstein's theory of general relativity proposes that the most densest and massive objects conceivable, such as black holes, have gravity that is so strong that nothing, not even light, can escape their grasp. So if light is not given off by black holes, how do we detect them? It would seem that attempting to detect a black hole would be "a bit like looking for a black cat in a coal cellar" (Hawking 121). Following are just four ways in which black holes can (and have been) detected.

When a star collapses and changes into a black hole, the strength of its gravitational field still remains the same as it had been before the collapse. Therefore the planets in orbit would not be affected. The planets would continue in their orbits as usual and would not be drawn into the black hole. Because black holes do not give off any light, the planets would appear to be orbiting around nothing. There is reason to believe that the planets could just be orbiting about a star that is too faint to be seen, but there is an equal chance that a black hole could be present (Hewitt 187).

Because the gravity of a black hole is so intense, dust particles from nearby stars and dust clouds are pulled into the black hole. As the dust particles speed and heat up, they emit x-rays. Objects that emit x-rays can be detected by x-ray telescopes outside of the Earth's atmosphere (Miller).

Black holes can be detected through a technique called gravity lensing. Gravity lensing occurs when a massive object, in this case a black hole, passes between a star and the Earth. The black hole acts as a lens when its gravity bends the star's light rays and focuses them on the Earth. From an observer's point of view on the Earth, the star would appear to brighten. Einstein's general relativity theory suggests that light should follow the path of bent time and space, which in this scenario, is bent by the black hole's gravity (Miller).

Black holes can be detected by measuring how much mass there is in a certain region of space. Black holes have large, dark masses concentrated in small volumes. If a region has large amounts of this dark mass, then one can suspect the presence of a black hole.