A black hole is a region of space in which the gravitational field is so powerful that nothing can escape after having fallen past the event horizon, even light is unable to escape from it. Light rendering the interior invisible. It is impossible to see a black hole directly because no light can escape from them, they are black. So black holes have never been directly observed. Black holes are presently understood, and are described by Einstein`s theory of general relativity. This theory predicts that when a large enough amount of mass is present with in a sufficiently small region of space, all paths through space are warped inwards towards the center of the volume, forcing all matter and radiation to fall inward.
When a large star has burnt all its fuel it explodes into a supernova. The stuff that is left collapses down to an extremely dense object known as a neutron star. If the neutron star is too large, the gravitational forces overwhelm the pressure gradients and collapse cannot be halted. The neutron star continues to shrink until it finally becomes a black hole.
A supernova occurs in our galaxy once every 300 years, and in neighboring galaxies about 500 neutron stars have been identified. Therefore we are quite confident that there should also be some black holes and it is now believed that at the center of each galaxy there is a super-massive black hole that is millions to billions of times heavier than our sun.
