Spacetime has a constant positive energy level, plus or minus uncertainty, which I will label ST. All other fields, including motion, draw their energy from spacetime. The total energy at any point in space = ST ± uncertainty. We will ignore the uncertainty, as it has little to do with what follows.
Mass is energy. No particle with mass can move at the speed of light, because doing so would require more energy than is available (ST). Photons do not have mass, and only travel at the speed of light.
Gravity is the gradient of ST.
A black hole is the place where total used energy = ST. There is no energy left remaining. There is no gravity inside a black hole, because the gradient is zero. This makes a black hole not a singularity, but a sort of discontinuity.
Each particle in a black hole has mass and energy. Their velocity cannot be entirely directed towards the center of mass, because black holes spin and move through space. And remember that there is no gravity inside the black hole, because the gradient, by definition, has been reduced to zero.
They are either a very compact mass or a hollow shell. It has been shown that all the energy/mass of a black hole can be contained in a shell at the event horizon. Every bit of energy added to the black hole makes the shell/event horizon just a tiny bit larger.
