According to quantum field theory, the vacuum (the quantum state of absolute nothing) is not empty, but teeming with particles - pairs of particles and anti-particles are appearing and disappearing all the time. If the partners of such a virtual pair are close to the event horizon around a black hole they may be separated as one falls into the black hole and the other escapes from the vicinity and are forced to materialize. The black hole is not completely black, but emits particles in this way.
Hawking radiation is characterized by a temperature that depends on the size of the black hole (the radius of the event horizon that is proportional to its mass). The smaller the black hole, the hotter it is, for the following reason: smaller black holes have stronger tidal forces than larger ones, and it is the tidal forces that separate the particle – antiparticle pairs.. Solar-mass black holes turn out to be about
times colder than the Cosmic Microwave Background, and supermassive black holes like the one in the centre of our galaxy are even colder.