crudely put: Even empty space has a certain amount of energy. This energy manifests in the form of “virtual particles”. These are particle/anti-particle pairs that spontaneously spring into existence and then almost instantly annilate each other.
The existence of these particles is so brief and the counter-balance of their forces and magnitudes so perfect that the net effect is almost as if they never existed, hence the term virtual particles. However, though fleeting, certain effects such as the Lamb Shift are thought to suggest the existence of particles such as virtual photons.
As a side note: if a particle/anti-particle pair forms near the event horizon of a black hole, one of the particles might fall into the black hole and be lost while the other escapes into the universe. Since the particles could not annilate as they normally would the net effect is that a particle springs into being out of nothing. In actuality the energy to form the particle comes from the black hole it’s self, thus decreasing the black hole’s mass.
This process, a form of quantum tunneling, when repeated over a long period of time will eventually cause even the inescapable mass of a black hole to gradually evaporate.