Black holes are essentially stars that explode, then collapse into themselves, due to their own gravity. this gravity is then intensified, by the newly formed object being a much more dense material than the star it came from. so, my guess is that in the centre of these black holes, are (originally) a superdense block of whatever the last elements of the star were. of course, it would be impossible to see this block, because of the effects it causes, but yeah, i believe there is a mass of heavy elements at the core. (the lightest elements were likely cast off by the original explosion, but then reabsorbed due to their proximity to the black hole. So. long term, whats in a black hole is whats chucked in there, makes me think they are nature's trash compactor... which is awkard, considering our solar system is revolving around one, at the centre of our galaxy.
