I was going to post pretty much exactly this.
Between dark energy and ordinary gravity, two comoving objects in an otherwise empty universe will inertially drift toward each other if they're below a certain distance apart, and away from each other if they're above that distance apart. (And sit stationary at exactly that distance, with the attractive or repulsive forces ramping up continually as they get further from that exact distance apart).
That sounds like it could very well be accounted for by a single force (or, more accurately, a single relationship between the distribution of mass and the curvature of spacetime) that's gets increasingly less attractive with distance and then goes to zero and then negative (repulsive) beyond a certain distance.
I don't know how that would explain away dark matter, though.
But thinking about it, I'm reminded of how Mach's principle holds that there really is no difference between a rotating bucket of water in a stationary universe and a universe rotating around a stationary bucket of water: the sloshing of the water to the outer edge of the bucket can equally well be explained by the frame-dragging of the rest of the universe spinning around the stationary bucket and water, as it can be by the inertia of the water spinning in the bucket.
Frame dragging is a gravitational effect, so if gravity does go zero or even negative at immense distances, then perhaps the frame-dragging of the rest of the universe spinning around a galaxy is less, which in the usual frame of reference, according to which that galaxy is spinning within the universe, would mean that the galaxy "feels" less of its spin, which would explain why its stars aren't flying out of it, without invoking any dark matter to hold it together.
I'm not sure how any of this explains the Bullet Cluster observation, though.