[VintageCool]

That doesn't actually solve dark matter.

We know that the rotational velocity of a stellar object is proportional to the mass inside its orbit and inversely proportional to its distance from the center.

From measuring the velocities of stars in other galaxies, we find that the stars distant from the core are orbiting much faster than we'd expect them to. The velocity as you go out from the center should drop off quickly; it doesn't.

Therefore we hypothesize that there is a lot more matter at the outer reaches of a galaxy than what we're seeing.

So... you need not just more mass, but mass that isn't clumpy; mass that is mostly at the outer reaches of a galaxy and not the center.

A massive black hole at the center just doesn't cut it.