Might suggest too that there are two supermassive black holes in the center of the Milky Way
It would take two to make the spiral waves. Perhaps galaxies that are not spiral galaxies only have one black hole in the center. So the Milky Way might have been born in a collision. And that might explain the spiral structure.
Non-luminous, ordinary matter further out might also explain the flattening of the angular velocity curve. I think exotic dark matter is bogus.