Even if you are correct about the galaxy rotation curves -- which everything I've read and heard leads to me believe is rather unlikely -- those curves are only one of several pieces of evicence that point to the existence of dark matter. The wikipedia page list many of these: https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
I only mentioned MOND as it is one of the more widely-known plausible alternatives that have been pursued.