If you enter Relativity into the equation, it's not hard to know what is inside a black hole.
If I understand Relativity correctly (and I admit this is not only possible but likely I do not) then time, as seen from the outside would come to a stop.
This means that everything that has fallen into the black hole is still AT the even horizon.
This would mean that the answer to the question of, "What is inside a black hole" is nothing.
There isn't even any space inside a black hole. If we took a trip into a black hole from our sister ship watching from the outside they would see us just sort of fade away, never actually reaching the event horizon, but WE would see something very different.
Assuming our star ship of the imagination could survive, we would see the event horizon coming to meet us faster and faster until we almost reach it, then it would vanish in a blinding flash of light but the universe around us would look very different, because we will have arrived in the very distant future.
We will have arrived at the day that particular black hole we tried to fly into evaporated complacently due to Hawking radiation, and for a stellar mass black hole, that day is very likely to be about 10 to the 100 years in the future.
There will be no stars, no planets, and likely no matter at all, aside from a few black holes more massive than the one used to get us here. And we'll have no way of getting back to our own time.
