As of 2010, the closest known black hole to Earth is V4641 Sgr. According to Tega Jessa of Universe Today, V4641 Sgr is located in the Sagittarius arm of the Milky Way galaxy and was first discovered in 1999. This black hole is 1600 light years from Earth.
Black holes are the last stage in a star's life cycle, forming when a star collapses under the weight of its own gravity, explains Jessa. They are discovered using radio telescopes, which detect radioactive emissions from matter that falls into them.
Based on Einstein's theory of general relativity, objects can fall or be pulled into a black hole, but they can never reappear. Because black holes are basically invisible, scientists rely on the observation of matter surrounding the possible location of a black hole.