Black holes are invisible regions of space where the force of gravity is so powerful that nothing, not even light, can escape.  There are three kinds of black holes - stellar, supermassive, and middleweight.  Scientists recently discovered the middleweight black holes by detecting their X-ray light using high technology, special telescopes, and trained astronomers.  A middleweight black hole is a black hole that is smaller than a supermassive black hole, yet larger than a stellar black hole.  Stellar black holes are the remains of a neutron star (a dense, tightly-packed star; once a huge star that shrank to a very small size) imploded to about two to three miles in total diameter or less, and then it becomes a black hole.  A supermassive black hole is the remains of many novas (the explosions of stars), or the remains of neutron stars.  A supermassive black hole has a mass (a measurement used to measure the amount of matter contained) equal to one million to one billion Suns.