It's a hypothesis, most of this stuff is, can't be proven, probably ever. It's a black hole, they reckon that an atom in a black hole is about 99.99% smaller in volume, compared to it's mass, which remains the same. My view is that the only thing that could have caused anything to happen in a singularity of infinite density in which time didn't exist was an action, such as the big crunch of the previous universe, black holes smashing back into the main one, left after the last big bang, with all the rest of the universe in them. This would be the force necessary to turn that motionless nothing back into fast moving energy. They say it was a quantum fluctuation, which is basically just them saying they don't know, but the big bang was originally taken as proof of the literal truth of Genesis, by the majority of scientists of the time, and religion played a huge role, particularly in this field. They suppressed Einstein's cycling universe theory, sent him death threats, and pretty much told him what they wanted him to say, which wasn't what he thought.