According to general relativity, all the matter that makes up a black hole is crushed together at a single miniscule point at the black hole's center.⁷
7. It is somewhat of a misnomer to speak of the "center" of a black hole as if it were a place in space. The reason, roughly speaking, is that when one crosses a black hole's event horizon – its outer edge – the roles of space and time are interchanged. In fact, just as you can't resist going from one second to the next in time, so you can't resist being pulled to the black hole's "center" once you've crossed the event horizon. It turns out that this analogy between heading forward in time and heading toward a black hole's center is strongly motivated by the mathematical description of black holes. Thus, rather than thinking of the black hole's center as a location in space, it is better to think of it as a location in time. Furthermore, since you can't go beyond a black hole's center, you might be tempted to think of it as a location in spacetime where time comes to an end. This may well be true. But since the standard general relativity equations break down under such extremes of huge mass density, our ability to make definite statements of this sort is compromised. Clearly, this suggests that if we had equations that don't break down deep inside a black hole, we might gain important insights into the nature of time. That is one of the goals of superstring theory.
"The Fabric of the Cosmos" - Brian Greene
