Black holes are difficult to find because they do not let out any light. They can be found when black holes suck in other stars. When black holes suck in other stars, the black hole lets out X-rays, which can be seen by telescopes. Hawking talks about his bet with another scientist, Kip Thorne. Hawking bet that black holes did not exist, because he did not want his work on black holes to be wasted. He lost the bet.