What is inside a black hole?
We actually don't know and may never know what is inside a black hole.
Soon after Albert Einstein published his field equations of General Relativity, Karl Schwarzschild found an exact solution to the equations.
The Schwarzschild solution has two singularities where the equations have infinite values. The first one defines the Schwarzschild radius which defines the event horizon of a black hole.
The vent horizon effectively separates the inside of a black hole from the rest of the universe. Nothing which passes the vent horizon, not even light, can escape. The implication of this is that we can never see what is inside a black hole.
It is theoretically possible to cross the event horizon but it is impossible to communicate information outside.
The second singularity is worse. It describes a point of infinite density at the centre of the black hole. Everything which enters a black hole will ultimately become part of the singularity.
Physicists try to avoid singularities. They can often be removed by a change of coordinates. The fact that there are singularities means that our physics breaks down.
We need need new physics to truly explain what is inside a black hole.