1) The vast majority of matter in the Universe is invisible. We've tried looking for it in a variety of ways for decades but we can't find it but we're sure it's there.
2) Our model of the Universe is slightly incorrect. It works in many many cases but not in interesting outlier situations. eg. at very low accelerations.
MOND doesn't explain all of the missing mass, it just reduces it to a fifth of what it was. So you need MOND and something else, which is two things, which Occam's Razor disfavors anyway.
> So you need MOND and something else, which is two things, which Occam's Razor disfavors anyway.
Misapplication of Occam's razor. LCDM also needs multiple things: non-interacting dark matter + a fine-tuned distribution of DM that cannot be a priori predicted from any observations, but only post-hoc fitted after observation. By contrast, MOND has successfully predicted rotation curves (and lots more) from the visible matter alone.
You can't naively apply Occam's razor to two theories that both fail some set of observations. However, as a scientific theory, MOND has a better track record of successful predictions.
I don't necessarily believe canonical MOND is the answer, just that it seems more reasonable to me to assume we don't have the Universe completely figured out and accurately modeled just yet vs. being emphatically sure most matter never interacts with light but only with gravity despite continued failure in finding direct evidence of it because it would satisfy our current model without the need to adjust it.
It took us until the 20th century to discover radiation, despite it having extremely direct and powerful interactions. Gravity is a shitty, weak interaction that we largely don't have a full understanding of, so why is it so unlikely we haven't properly discovered matter that only interacts weakly with gravity? How the hell else would we "discover" it?
1) The vast majority of matter in the Universe is invisible. We've tried looking for it in a variety of ways for decades but we can't find it but we're sure it's there.
2) Our model of the Universe is slightly incorrect. It works in many many cases but not in interesting outlier situations. eg. at very low accelerations.