There are several strands of evidence which lead to the belief in the existence of dark matter. It's not just observations of the dynamics of galaxies (which is

There are several strands of evidence which lead to the belief in the existence of dark matter. It's not just observations of the dynamics of galaxies (which is probably what most people have heard of), but also calculation of the amount of baryonic matter made in the big bang. So alternative theories like MOND may explain galaxies but have nothing to say about the big bang. The potential existence of dark matter candidates, which some suspect to be sparticles predicted by Supersymmetry (maybe the LHC throws something up there or, as always, it's just the next collider away), is another reason to pursue dark matter ideas.