> Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
For some general misunderstanding to explain what we see, ie something which is not dark matter, it means our equations of gravity must be wrong.
People have tried hard to tweak the equations to fix the observations. One such proposal is indeed MOND that the submission is about.
The problem with tweaking the equations is that we have a lot of observations of very different circumstances and scales, and it's so far proved impossible to tweak the equations in a given way so they explain all of the observations. What happens is that you can tweak them so they fit one set of observations, like MOND fitting galaxy rotation curves, but then not fit other sets of observations, like MOND with the motion of galaxies in galaxy clusters as discussed in the submission.
Meanwhile, adding dark matter seems to work much better at explaining all the observations at once. I say much better because there is still some tension, but it's way less than any modifications of gravity.
Having dark matter particle(s) is also not that strange. As far as we know there's no requirement that a particle interacts with any of the known forces beyond gravity. And while the early particle candidates seem to be getting ruled out, axion-like particles, originally proposed to solve entirely different problems, are still a good dark matter candidate.
That said, it's not settled. People are proposing alternatives. One example I know of is Alexandre Deur[1], a QCD physicist which has taken his QCD knowledge into the realm of GR and proposed that perhaps graviton-graviton self-interaction can explain a lot of the dark matter phenomena. It's not mainstream, but he's getting published in peer-reviewed journals.
[1]: https://arxiv.org/search/gr-qc?searchtype=author&query=Deur,...