We know - or theorize - that a spinning black hole drags spacetime in its surroundings, producing measurable effects on the accretion disk (the minimum stable orbit radius). What I cannot grasp is why this isn't the case for a rotating galaxy. Why the mismatch in classical rotational velocity predictions isn't explained from the spacetime dragging of the total mass of the galaxy at a given radius value? Why do we need dark matter to adjust for the "strange" measurements?
The spacetime dragging is only significant very close to the event horizon of a spinning black hole i.e. within a few Schwarzschild radii. While in principle the effect exists for a rotating galaxy in practice it is so small as to be completely undetectable.