If you changed either the special theory or the general theory just slightly, dark matter would disappear completely, I suspect. Actually, you would probably need to change both because they dovetail with each other.
In particular, I note that the evidence for dark matter and dark energy are contradictory. On a galactic scale, it seems that there is too much gravity to be explained by the visible matter, so they hypothesize dark matter. But on the universal scale, it seems that the expansion of the universe is too fast, given how weak gravity is, to be explained without dark energy. So on one hand, gravity is too weak to explain reality. On the other hand, it’s too strong. In my opinion, that shows that it tapers off at a rate that is greater than what we believe.
I recently heard on the John Batchelor show that an experiment to detect dark matter on a galactic scale failed to do so. It may well be that dark matter does not exist and we don’t fully understand physics yet.
Scientists tell us that the universe is expanding. Well, not only expanding but increasing the rate of expansion. It's expanding faster (accelerating).
Theory goes that when mass accelerates towards the speed of light, gravity increases and the material gets more massive, requiring ever increasing amounts of energy to accelerate less and less.
No one has calculated this energy investment.