I think about that sometimes. Haven't read a lot on the subject, but I imagine all those particles still exist, yet they are incredibly compressed. It's impossible that they don't exist, since all that mass is still there( otherwise I imagine that transforming all that mass-to-energy like in E=MC^2 would blow up several galaxies). Hah, if you think about it, probably black holes exist to prevent the Universe from blowing up! It's a sort of astronomical garbage-collection.
But something new comes to my mind: what if black holes are actually a massive version of a sub-atomic particle? Since all those particles collapse in a point, maybe they don't coexist, because they "don't have the space". So the big particles are broken in elementary particles, until, at some point, just those basic energy vibrations remain, which unite into a single, dense ball of energy. I read somewhere that if you'd take all of the space from within the atoms, a cruiser would fit in a space less than the tip of a needle. Maybe that's what's happening with black holes :.