Observations on galactic scales reveal a huge discrepancy with General Relativity, a classical gravity theory. I begin by describing why the fundamental assumption of classical gravity theories -- that quantum mechanics is unimportant -- likely fails in galaxies. Thus, I introduce an empirical modification to gravity. This 30 year old theory (due to Milgrom) predicted many of the observed properties of galaxies very well. It also predicts an ancient flyby of Andromeda. I present recently gathered evidence, some of it my own, which suggests this really did happen. Attempts to explain these observations in the standard model - where General Relativity is saved by hypothetical invisible matter - run into severe difficulties. So far, these can be resolved very naturally by abandoning dark matter in all galaxies. Moreover, 30 years of null detections in direct searches for dark matter and recent LHC results argue against its existence and have ruled out dark matter with certain properties. Still, one day, it may be found. But it may not.
