The No-Hair Theorem

Black holes are described by just three properties: mass, electric charge, and angular momentum (spin). Nothing else. Two black holes with the same mass, charge, and spin are identical regardless of what fell in to create them. This is the no-hair theorem — black holes have no distinguishing features beyond these three numbers.

This radical simplicity is both elegant and disturbing. A black hole made from a collapsing star and one made from collapsing furniture would be indistinguishable if they shared the same mass, charge, and spin. Everything else about their history is erased.