[matthewdgreen]

I’m not objecting to the idea of alternative theories. I’m objecting to the parent commenter’s idea that “explains more galaxies with a single parameter” should be an argument in favor of it. By that metric your Mercury example is a great illustration: oddities in several of the outer planets’ orbits were caused by unknown planets, but only Mercury’s unusual orbit requires new theories of gravity. So clearly the Newtonian theory explains more instances with a single parameter, and is thus better. (Obviously in the real world this is not how theories should be evaluated.)

The appropriate MOND/DM analogy here would be to propose general relativity as an explanation for Mercury but then find another planet(oid) in the system where GR suggests an effect, but to notice that the effect is measurably not present. You can’t just say “got most of them right” when you propose such a theory. You need to revise your theory until it explains the observations.