We can only theorize what's inside of a black hole; especially at the very center of the black hole. Some are still skeptical towards the idea of black holes; but hopefully we'll have the tools soon to be able to observe them.
Physicists call the center of a black hole a singularity.
That is, a point in the black hole where matter is crushed into an infinitely small point as density becomes infinitely large.
Trying to imagine this is pretty difficult, though we have equations that point in that direction.
An analogy for a black hole is to think of our own star as a bowling ball sitting on a sheet of elastic (rubber). The bowling ball will make the rubber stretch downward some finite distance depending on the weight of the ball. And any smaller balls (planets) around the bowling ball will rotate (orbit) around the bowling ball.
Now a black hole will be so heavy that the rubber below the ball would be stretched infinitely downward so as to create a sort of bottomless hole. That is why is acts like a vacuum even to something as fast as light itself. This is the concept of a black hole and what's inside at the center.