Black holes are not made of matter… the matter has collapsed into pure energy. The form of energy is a mystery (it’s inside the event horizon) but since the gravitational field persists, it’s often referred to as pure gravitational binding energy. I graduated physics at Manchester Uni and I’ve still got a ‘preference’ to be as correct as possible when talking about BH’s and what they’re ‘made’ of. Kip Thorne also often refers to the stuff BHs are made of as ‘gravitational binding energy’ so I thinks it’s safe to do the same.
