Question

what inside of a black hole​

Answer 1

Answer:

According to General Relativity, black holes are empty. Completely empty. All of their mass is predicted to be concentrated into a dimensionless point called the singularity.

Now, this is probably not the whole truth, this prediction (and other reasons) is why we suspect that GR is incomplete and can not correctly describe the innermost core of black holes. But up to some inconceivably tiny “central” core, the inside of black holes is surely indeed empty spacetime, only that it is extremely distorted, and that is one of the reasons why it’s hard to talk about “the innermost central core of a black hole”, because spacetime there is so distorted that our usual concepts of distance or spacetime coordintates (from which concepts like “size”, “volume” or “density” can arise and make sense) likely lose their meaning and cease to be applicable there.

Some theories suggest that just before reaching the state of singularity, the space dimensions become more time-like and the time dimension becomes more space-like, a bit like having spacetime turned over like a sock, so that the “extension along which all the black hole mass sits” is not a spatial extension but a time extension, that mass has ceased to occupy space because it has become occupying only time, up to the infinite future. Close to a black hole we are receiving gravitational influence not from a location in space, but from the future. But this is speculative.