Distinguishing dark matter from modified gravity

Dynamical mass discrepancies in galaxies have two possible explanations: the existence of large amounts of dark matter or the breakdown of Newtonian gravity. True dark matter halos of galaxies could differ in several respects from apparent halos inferred from misinterpreted modified gravity, and we discuss two observational tests to probe these differences. Modified gravity gives generic, geometrical predictions for the shapes of apparent halos, and also requires that the inferred excess mass strongly correlates with the visible baryonic mass. Thus modified gravity is more easily falsified, but we find both hypotheses to be consistent with the meager data on galaxy and cluster halos; dark matter is not required by current data. We strongly encourage the acquisition of more data, which could ultimately eliminate one or other of these possibilities.