The point is that a galaxy acts sort of like a solid. The gravity appears to fall on inside that galaxy in the vicinity of 1/r. One explanation (the one that most astronomers believe) is that there must be a tremendous amount of matter in the galaxy that only weakly interacts with matter. We already know of these types of particles (neutrinos) exist so it is not a stretch to imagine more massive particles that only very weakly interact with matter. It would be much more surprising if the issue turned out to be gravity itself, since we to not see any sort of issues on shorter distance scales. You can believe what you want but my money is on dark matter.