The black hole is a region so dense that nothing, including the light can not escape its gravitational field. This is due to the evolution of extremely large stars, which begin their evolution as giants and supernovas, whose star nucleus, if large enough, continues to countract and form neutron stars. If the contractions continue, all around the neutron star, including the neutron stars themselves, will pulled into what is called the horizon of black hole. Beyond the horizon, the expansion stops and the matter no longer pulls in.