Many apologies in advance if this question is ridiculous or if it has already been answered on another thread. I've searched and searched through the forums and haven't found the answer - please do direct me accordingly if that's possible. If not - please help! Preamble: We know from calculating Galaxy rotation curves that the visible mass in spiral galaxies is too low to be consistent with the observed orbital rotation velocity of galaxies. Introducing the notion of dark matter to provide the "missing mass" solves the problem. Calculations demonstrate that dark matter would need to exist in a mass ratio of 5:1 to visible matter to explain galactic orbital rotation velocities. In Special Relativity the mass of an object increases with velocity. According to the relativity calculators suggested by Marcus a relative velocity of 0.98c would increase the mass of an object by approx 5 times. Question: 1) Does this velocity-mass relationship apply in general relativity? If so . . . 2) Would we expect to observe differences between (a) a model of a galaxy "at rest" with a dark to visible matter mass ratio of 5:1, and (b) a model of a visibly identical galaxy with velocity 0.98c (relative to our observational point of reference) and no dark matter? Thank so much in advance for your help.