Black Holes Feed On Quantum Foam, Says Cosmologist — The Physics arXiv Blog — Medium: “Quantum ﬂuctuations in the form of mini black holes can couple to macroscopic black holes and allow the latter to grow exponentially...”
He says this kind of growth can easily account for the observed size of relatively young supermassive black holes. And he goes on to predict that this mechanism could allow smaller black holes to grow too. In fact, he suggests these smaller objects, which would be difficult to observe from Earth, could make up a significant fraction of the mysterious dark matter that astronomers believe the universe must contain.
...He says that this quantum feeding mechanism must apply to the supermassive black hole at the centre of our galaxy and that consequently, it must be growing at the rate of about 10^-3 solar masses per year.