None of this explains the galaxies which don't appear to have any dark matter or deviations from Newtonian gravity. If there were any such deviations, then we should not find any galaxies which do obey ordinary Newtonian gravity by not having any dark matter in them. The only thing these galaxies have in common is that they are sparse and spherical in shape. Now, it just so happens that the "shortcut" used in all gravitational calculations is to "pretend" that all the mass is centered at a point in the middle. This works well for the solar system, where most of the mass really is in the center, but doesn't represent a disk. Suppose the galaxy were in the shape of a flat square? How would you apply the gravitational formula when it has no radius? You cannot. You also cannot apply the shortcut to a disk shaped galaxy. If you do the calculations, you find that even though the quantity of matter decreases exponentially, it still DOES INCREASE. Which means the mass inside the orbit constantly increases and the gravity always gets STRONGER! Always stronger which means increased orbital velocity the further you go out. You would definitely not expect the force of gravity to decrease. That is the difference between a disk shaped galaxy and the solar system. The "shortcut" doesn't work for a disk shaped galaxy, but works great for the solar system. Now, why do spherical galaxies, not have any dark matter? It's because the spherical galaxy does obey the gravity "shortcut" due to the required spherical symmetry and we would "predict" that any spherical galaxies would obey normal Newtonian gravity using the "put everything in the center " shortcut. So no need for any modification of gravity, If you do the calculations and manually integrate all of the mass bit by bit, you will see that spiral galaxies behave exactly as we expect when applying the "actual" Newtonian gravity law which only applies between 2 distinct masses. Anytime you see an "r" for radius, you are NOT dealing with the fundamental Newtonian gravity formula. Once again, try to apply that to a mass which is in the shape of a flat square.