More clarification
IMO the orbits of stars around galaxies ARE obeying Newton's law of gravitation, but because we can only SEE the stars, and those stars are concentrated in the inner disk of the galaxy, we are not able to show Newtonian gravity. We can't SEE the outer halo dust and gases that are orbiting much slower, as predicted by Newton's gravity. I think extra-long exposure photos of galaxies, maybe in the hydrogen or helium lines, would show a vast halo of matter around each one, extending much farther than the visible disk.