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Most black holes form from super-massive stars that explode (supernova) at the final stage of their lives. The outer layers explode out while the inner core compresses under gravity. As matter becomes denser, the gravitational force becomes stronger and so compresses the matter further and so on. If the density becomes high enough, then not even light travelling at 300,000km/s cannot escape. The object itself as well as a surrounding area is completely void of light - A Black Hole.
How can they be detected if space is dark too? One method is to observe light being curved around the black hole. A prediction by Einstein is that gravity will bend light around the object. In 1919, stars were photographed near the sun during a total solar eclipse. As Einstein predicted, the stars have 'moved' slightly from their normal positions due to the Sun's gravity bending their light around the sun itself. The stars haven't moved, only their images have. A black hole will distort the light even more.
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In theory, two black holes orbiting each other can loose orbital energy, get closer together and eventually merge into one. It will not create any explosion but will send out a massive gravity wave across the Universe. As of June 2016, two have been discovered via Gravity Wave detectors.