A black hole is a point is spacetime that has such a strong gravitational pull that nothing can escape. Anything that comes near a black hole will be sucked inside. So if you want to know what is inside a black hole, the answer is everything and anything.
There are theories that our universe originated from a black hole, so everything you see and have ever seen was once inside a black hole. It was just so dense and compressed that it did not look like the matter we see around us today. A black hole is black not because of what is inside but because nothing, not even light can escape.