Thats a hard question! A black hole isn’t really a hole as such, it is an extremely dense dead star that attracts all matter (things in the universe). At it’s center, a point of singularity, all the matter that falls into the black hole is compressed into this infinitely small sized place and will just sit there for a very long time. So I guess you could say that at the ‘bottom’ of a black hole, we find all this compressed matter that the black hole has sucked in. Hope that helps 🙂
And black holes don’t just suck in matter. They suck in all light that goes near it! That’s why it’s black, it has no colour because all the light in or around it is sucked downwards.
Where does it go?! Well, some people believe that black holes actually create worm holes from one place in space to another. Or one place in TIME or another. Could this mean time-travel? Probably not, as we wouldn’t be able to survive the crushing gravity of a black hole.