Sometime after Karl Schwarzschild discovered a solution to Einstein’s general theory of relativity, which predicted the existence of black holes, a problem had arisen. As far as scientists could tell, black holes seemed to violate the second law of thermodynamics, which states that heat will always travel from
warm bodies to cold bodies and never the other way around. Another way of describing the second law is that within a system, entropy, or disorder, will almost always increase. This was a problem for black holes. Anything that enters the black hole cannot escape and is eventually crushed to a minuscule point at the black holes centre. How then could entropy increase? This problem was later solved by none other than Stephen Hawking.
A well-known feature of quantum mechanics is that antiparticle pairs can spontaneously come into existence, stick around for a little while, and then bump into each other and annihilate. For example, an electron and a positron can spontaneously pop into existence, stick around for an extremely short period of time, and then bump into one another and annihilate. This idea is important for understanding how the second law applies to black holes.
Beyond the event horizon of a black hole this process of antiparticles coming into existence and annihilating one another happens normally. If, however, the particles form sufficiently close to the black holes event horizon, one particle can get pulled in while the other escapes. Hawking found that the positive particle escapes the black holes gravitational pull while the negative particle falls inward. When viewed from far away, the positive particles escaping appear as radiation being emitted by the black hole. This radiation was called Hawking Radiation. The negative particle that falls inward causes the black hole to evaporate. You can think of this like a burning piece of coal. The Hawking Radiation is the smoke being emitted, and over time the piece of coal burns away. Much like how, after many trillions of years of having negative particles falling inward, the black hole begins to decrease in size and eventually disappear entirely. Entropy does increase for a black hole. by astrophysics_