[henkster]

So what is a black hole composed of? Is it still essentially neutronium, but with sufficient gravity that it has an event horizon? Or does the matter become so compressed that it becomes "something else?"

[17th Miss Regt]

Is it still essentially neutronium, but with sufficient gravity that it has an event horizon? Or does the matter become so compressed that it becomes "something else?"

It becomes something else. The neutrons are smooshed together in the collapse.