I would like to ask if there is any direct evidence of black holes with spin. I understand that there are formulas that would describe a black hole if it were to have spin, but this does not prove that they actually can. I also know there are accretion disks and relativistic jets around black holes. However, those only prove that matter revolves around black holes and that sometimes, if the conditions are right, matter and energy can be directed away from them in jets. In simpler terms, accretion disks and relativistic jets only decisively prove that there are accretion disks and relativistic jets. Is there any solid physical evidence of a black hole rotating?