>>6230063While you brought up the case of Mercury and how new gravitational theories were needed to explain its orbit, consider the similar case of Uranus. It too had an orbit that didn't match Newtonian predictions, leading astronomers to theorize a "dark planet" that was altering its orbit. Said "dark planet" was discovered 25 years later; we call it Neptune.
Both modified gravity theories and dark matter should be given serious consideration, but so far the dark matter theories make more sense. For example, there is the bullet cluster, where the observed matter doesn't match the gravitational lensing going on. Or that galaxy you mention that doesn't seem to have dark matter - ask yourself what makes more sense, gravity behaving differently in one galaxy than all the others, or one galaxy not have as much of a particular kind of matter? There are a number of other observations that make far more sense with dark matter than a modified gravity theory.
