What is the type of matter that constitutes a black hole? Or is it matter at all? If the super dense material is continually being compressed where does it finally end? Surely there must be a limit or is it squeezed into energy?
The matter that falls into a black hole is no different from the matter which makes up the rest of the universe. However, physics as we currently understand it breaks down at the centre of a black hole.
We think we understand what happens in extreme gravity (that's General Relativity), and we think we understand what happens at microscopic scales (that's called Quantum Mechanics/Physics), but if the two are combined the theories break down and we have no idea what's going on.
Only a theory of gravity that is compatible with quantum mechanics (often called quantum gravity) could describe the physics "inside" a black hole. Currently, there is no such theory even though several physicists around the world are working on it. Until they (or others) come up with an answer, we will not know what exists at the center of a black hole.
This page was last updated June 27, 2015.