Infinite density, that's what they're talking about, which basically means it couldn't be any more dense, it's packed as tightly as it can be. The electron collapses and stops spinning around the nucleus, taking away an estimated 99.99% of the volume, but not the mass, so it has massive gravity, in a small space, but from a distance, it still has the same gravitation pull. The difference is, when you go into it, there's no coming back out, for anything, and I believe that includes dark energy, even though that's a completely unknown thing.
That's the force that is supposed to be pushing the expansion of the universe, but they only assume it's there because the universe appears to be speeding up in expansion. I reckon it's just the subatomic particles bouncing around, pushing against each other causing outward pressure and also pulling the universe with their own gravity. I believe all forms of energy have gravity, and that they've already fallen into the massive black holes that exist now, such as the ones in the middle of just about every galaxy, millions of stars in mass.
So, rather than continuing to push the expansion, my theory is that this energy, dark energy will add to the mass and gravity of the black holes, the stars will burn out, turning into more black holes, which will then form orbits and fall into each other, with black hole cluster galaxies forming and moving into smaller and smaller denser regions of space where all the mass is, and in between these places, absolutely nothing, so gravity will be the only force left, causing the collapse, and when huge black holes containing all the mass/energy in the universe smash together at the speed of light, big bang.