Before the middle of 1970, scientists always thought that black holes only sucked matter and were "dark" around them. When more matter is attracted, the mass of the black hole will also increase. Hawking's other original idea is to point out that black holes are not really black, and black holes have temperatures. Substances with temperature will naturally release radiation. Because radiation is a kind of energy, black holes will eventually disappear because of energy exhaustion, which is the so-called "black hole evaporation theory".

Hawking made this discovery thanks to quantum mechanics. In the past, scientists thought that the "vacuum state" in a black hole meant that there was no matter inside, but Hawking pointed out that if quantum mechanics was correct, when people looked at the "vacuum" of the universe from an extremely microscopic perspective, they would find that pairs of photons were constantly generated, but the energy in the vacuum could not be kept at a fixed value, so that these pairs of photons kept shaking and eventually disappeared. He believes that the smaller the mass of the black hole, the faster the photon release, which will accelerate the disappearance of the black hole.

This kind of energy released by a black hole is called Hawking radiation, but because this kind of radiation is extremely weak, although the scientific community has realized that it is feasible in theory, it has not been found from observation. This also shows that even if Hawking radiation is real, its energy is too weak, which means that it takes a long time for black holes to run out of energy and cause evaporation.