It's not as developed as it could be..... but it goes like this:
Maybe the odd behavior of stars on the outer rim of a galaxy could be the result of delayed propagation of gravity? If the star in question is say.... 10 light years out from the center, then maybe it still perceives its own relative location to the core to be where it was 10 years ago?
I'm not sure how much that would affect things, but it would change the vector of the gravitational force acting on it. Instead of being at a 90 degree angle to its current direction of motion, gravity would appear to be pulling from a slightly smaller angle in front of that motion.
Another complication that this would introduce is that the gravitational attraction from objects closer to the star (like other stars in the same galaxy) would be subject to this effect to a smaller degree, because information about changes in their relative position is arriving earlier.
Of course.... this might not be insightful at all. Maybe its already being accounted for and we *still* get dark matter.