What is the temperature inside a black hole?

Dominic - It's actually incredibly hot...

At least, the surface of the black hole is very hot, because, as material is falling in towards the black hole, it's being compressed into an ever smaller volume of space: there's less space close to the black hole than there is far away from it and so that material is elevated to tremendous pressures and tremendous temperatures of hundreds of millions of degrees, which means it doesn't glow red hot. It glows hot in ultraviolet and x-ray light.

And that is how we can detect that black holes are there, because we can see this very hot material producing x-rays, with x-ray telescopes.

Once it's actually past the event horizon, it's very difficult for us to say what happens to that material beyond that point, because we can't see it and we don't fully understand what laws of physics apply inside the black hole itself.