It doesn’t work out because then you would expect the radial mass distribution of a galaxy to be just a multiple of its observed baryonic mass distribution, rather than almost exactly what you would expect from the gravitational collapse of a thin ‘gas’ of particles that cannot self-collide.
It doesn’t work out because then you would expect the radial mass distribution of a galaxy to be just a multiple of its observed baryonic mass distribution, rather than almost exactly what you would expect from the gravitational collapse of a thin ‘gas’ of particles that cannot self-collide.