Green dwarf said:
TL;DR: By a clock outside the event horizon, things slow down as they approach the event horizon and never quite reach it. From an outside perspective, there shouldn't be anything inside a black hole. Everything should be just outside the event horizon.
Reference:
https://www.physicsforums.com/forums/astronomy-and-astrophysics.71/post-thread
By a clock outside the event horizon, things slow down as they approach the event horizon and never quite reach it. From an outside perspective, there shouldn't be anything inside a black hole. Everything should be just outside the event horizon. There shouldn't be a singularity.
There are already a lot of threads on this. The Schwarzschild black hole is an "eternal" black hole, which is a solution to the Einstein Field Equations in vacuum. There is nothing anywhere in that model. There is only spacetime, and all vacuum with a certain spacetime geometry.
We can add a test particle to that model, which falls into the black hole and through the event horizon in finite proper time. Below the event horizon it has a further finite proper time before the model breaks down and the particle literally runs out of time. Note that the "centre" of such a black hole is not a point in space, but more like the end of time. And, "singularity" means where the mathematical model breaks down.
From that point of view, it makes no sense to say there isn't a singularity. The singularity is the (apparent) incompleteness of the mathematical model.
In addition, a star that is sufficiently massive will eventually be unable to resist total gravitational collapse, and forms a stellar black hole. It's true that beyond a certain point in the collapse, light from this process cannot reach a distant observer. Just because a distant observer can never observe an event does not mean that the event does not happen. The first thing to learn in relativity is that space and time are combined into a spacetime continuum; and, that there is no absolute frame of reference in which all things can be described fully. Your clock does not represent absolute time for the whole universe.
What happens to the matter in a collapsing black hole is unclear. The General theory of relativity has a mathematical breakdown and can't be the full story. Again, the singlarity is not a physical point but more like the end of time and represents a theoretical/mathematical (rather than physical) breakdown.
It's hoped that a quantum theory of gravity will more fully explain what happens below the event horizon for a stellar black hole. And that such a theory would be complete and not break down mathematically below the event horizon.