What is inside a black hole?
Generally an awful lot of ‘stuff’ in a very very small part of the universe.
Black holes (at least the ones you & I have heard of) grow by accretion - the process of tearing apart stars into dust and gas, heating them till they glow red hot, then white hot, then x-ray hot.
The plasma that results, is then compacted down till it occupies unimaginably small volumes, forcing electrons into the nucleus, then the resulting neutrons into a quark-gluon plasma.
A shorter answer may have been, we have no idea. All our models of matter break down at this point.
Anything that enters a black hole can never return, so it is impossible to see what is inside a black hole.
The event horizon of a black hole is a form of singularity. If you were to watch an object falling into a black hole you would see that the object experiences ever increasing time dilation. It would also appear more and more red shifted until it disappeared.
At the event horizon itself time stops. Nothing can escape from the event horizon, not even light.
There are theories about what would happen if you were in a spaceship entering a black hole. It would cross the event horizon without noticing it. This is assuming that the ship survived tidal forces.
Theories predict that there is a singularity at the centre of black holes. This is a point of infinite density. Anything entering the black hole will ultimately be destroyed at the singularity.
In reality our laws of physics break down when trying to describe a black hole. New physics may explain the interior of a black hole. Until then we have no way of finding out.