In 1993 Stephen Hawking proposed in
Black Holes and Baby Universes that there might be “primordial black holes which were formed in the early universe that could be less than the size of the nucleus of an atom, yet their mass could be a billion tons, the mass of Mount Fuji. A black hole weighing a billion tons,” Hawking explained, “would have a radius of about 10-13 centimeter (the size of a neutron or a proton). It could be in orbit either around the sun or around the center of the galaxy, emitting hard gamma rays with an energy of about 100 million electron volts.”