I. Mass bends spacetime. In the proximity of mass, points separate in space draw closer together (i.e. a distant observer watches stuff fall toward the centre), and points separate in time pass slower (i.e. the same distant observer watches physics slow down). This phenomenon is known as "gravity."
II. Gravity is pretty weak compared to the electromagnetic force. If the mass isn't that dense, then spacetime keeps flowing inward but the atoms inside spacetime bump up against each other. Their electrons stop each other from falling. This phenomenon is known as the "ground."
III. The ground has a limit to its strength; gravity does not.
IV. If the mass is denser, then atoms collapse into themselves, and the protons and electrons merge into neutrons. But gravity is even weaker compared to the strong nuclear force, so if the mass isn't super-dense, then spacetime keeps flowing inward but the neutrons bump up against each other. Their quarks stop each other from falling. This phenomenon is known as "neutronium."
V. Neutronium has a limit to its strength; gravity does not.
VI. If the mass is even denser, then weird shit happens. There's no physical mechanism which lets quarks collapse in on themselves, but there's also no physical mechanism which lets them not collapse, because the quarks beneath them are falling even faster. The quarks at the centre keep falling into the centre even though they're already at the centre. There is no more up; there are only different speeds of down. This phenomenon is known as a "black hole."
VII. Even for distant observers, black holes still have a measurable size, mass, rotation, and duration within spacetime. Weirdly, the size is proportional to the square root of the mass, instead of the cube root like normal matter. It's as if the whole mass gets smeared out onto an oblate spherical shell, with nothing on the inside. This kind of makes sense if you think about a distant observer watching physics slow down: at a black hole boundary it slows down all the way to zero, so you can never actually watch anything go through that shell, only get shellacked onto the outside. This phenomenon is known as a "firewall," and physicists are still debating whether it's real or an observational artifact. I prefer to think of it as "weird shit."
VII. Logically it is ridiculous to suppose that spacetime keeps falling forever into whatever is at the centre of mass. We have conservation laws. How can spacetime just keep flowing out of the universe? So there are theories about black holes being wormholes to white holes that spew matter outward, or about white holes being other universes' big bangs, or about spacetime just kind of getting denser and denser as it rotates around the centre, because gravity might have no limit but neither does centrifugal force, or about spacetime getting stuck-for-real in the firewall until the black hole evaporates, or about gravity being mediated at tiny scales by a quantum particle that spins all the way around every 180 degrees. All of these theories are supported by viable mathematics. None of them has been proven. This phenomenon is known as "theoretical physics."
VIII. In order to preserve any semblance of conservation of spacetime, something has to be holding up the universe at the centre of a black hole, whether it's centrifugal force, or a firewall, or an equal-and-opposite white hole at the big bang of our own universe, or whatever else.
IX. Physicists can sometimes get lost in the math and miss the obvious.
X. The dude holding up the inside of a black hole is obviously Atlas. I will not be taking questions at this time.
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