Question: what is a black hole made out of?

Louise Johnson answered on 16 Jun 2010:

It’s made of a huge star that has burned out. As it cools down, the gas it’s made of loses pressure (like a balloon in the freezer!) so the star shrinks and gets denser and denser, and its gravitational pull gets stronger and stronger. If it’s above a certain size, eventually it collapses.

If it’s a fairly small star, the atoms squish together and the star becomes a sort of soup of ions and electrons – that’s a “white dwarf”. Bigger than that, the electrons soak into the protons and turn them into neutrons – that’s a “neutron star” – and if it’s even bigger than that, all the matter falls into a single point that’s got such strong gravity that not even light can escape from it.
Nobody knows how the laws of physics apply there, and of course you can’t see what’s going on because no light comes out.

(Disclaimer: I only did one astrophysics course ever and that was in 1993 so I may not have everything 100% right! But I think that’s the general gist.)