This one has been bugging me, so I'm hoping someone can help me through it (I also think it would make a good question to answer on Astronomy Cast).
How does a black hole rotate? I can visualize how objects like planets and stars rotate because they have a finite spherical shape and whole thing is in rotation about the center of mass. But a black hole is a singularity. It's entirety is the center of mass. I understand a little about frame dragging and how a rotating object drags spacetime with it, but again I can't wrap my head around how a singularity could achieve this. I can't even understand it from a conservation of momentum perspective, such as a rotating star collapsing into a black hole, because the distances of any mass in the star to the center of mass goes to zero when the black hole is formed.
If anyone could help me understand, I'd appreciate it.