Question: What is inside a black hole

Simple answer is ‘lots of very dense mass’ otherwise it wouldn’t be a black hole.

I prefer this answer though…I’m going to have to give you a bit of a flippant answer here, because I’d better keep the answer fairly short.
Here’s a thought experiment and question for you: does anything from outside ever get inside a black hole?
Let’s assume that by ‘inside’ we mean ‘past the event horizon’.
Let’s assume that this situation is being viewed from significantly ‘outside’ (a long distance from) the event horizon.
Let’s consider the effect of time dilation (an effect theorised by Einstein and for which there has subsequently been plenty of evidence).
Consider The situation of an object falling into the black hole under the influence of the black hole’s gravity. Science fiction often describes the object accelerating into the black hole.
But, The closer that an object gets to the black hole, the stronger the gravity and therefore the stronger the time dilation. An object very close to a black hole event horizon would appear, due to the time dilation effect, to us to be moving very slowly and decelerating as time dilation increases.
From our point of view, the object will appear to never reach the event horizon, and would appear to just stop on the edge.
(From the object’s point of view, the rest of the universe speeds up and the object accelerates towards the event horizon).