True enough, but brightness isn't necessarily a good indication of where the mass is. Spiral galaxies, in particular, have enormous amounts of dust in and between their visible arms. The dust in the arms both adds to the mass there and makes them dimmer. The centers are mostly dust-free. This was known even before accurate measurements of velocity distributions, and is accounted for when astronomers complain about "missing mass."
The great thing about FR is that, no matter what the subject or the level of detail, there's always someone who can relevantly expand upon it. I dumbed it down, probably unnecessarily, for the sake of clarity. It is true that the stars in the galaxy were never expected to move in exactly a central potential.