Is there evidence for the speed of gravity? Is it an assumption that the speed of propagation of gravitational waves is exactly the same as the speed of light? If the speed of gravity is only slightly less than the speed of light, the effect might not show up over solar or intra galactic distances, but over extragalactic distances could it give the appearance that there is dark matter influencing motion?
It has long been one possibility that gravity does not work as in general relativity, or as in the Newtonian limit that is relevant to the gravity in a galaxy.  Thus, many different modifications to gravity have been tried.  But the reason they are generally not very successful is that dark matter seems to alter gravity on many different scales, from within a galaxy, to over galaxy clusters, to the expansion of the universe as a whole.  Thus it generally requires multiple different modifications to fit them all, and that makes for a very clunky model of gravity-- one can fit anything if given enough free parameters.  That sounds like it would be the problem for trying something based on the speed of gravity-- the timescale for propagation of changes would be very different over the three scales I just mentioned, yet dark matter has a similar influence on all three scales.