Galaxies don't behave how they should with our understanding of gravitational mechanics. The only thing that fixes the model is the addition of a lot of mass th

Galaxies don't behave how they should with our understanding of gravitational mechanics. The only thing that fixes the model is the addition of a lot of mass that we can't see. That would be easy enough to explain away as an incomplete model, except we have observed galaxies which do fit our models. The only way to explain that is if those galaxies are missing this invisible mass. There's no other way to reconcile these observations. How would you square these observations? Under our models of gravity, most galaxies behave as though they contain much more mass than what we observe. However, some behave exactly as Newton would predict. The misbehaving galaxies can be made to fit the model by adding mass in very particular locations, mainly as a halo or shell around the outside. We can show how much mass and where it must be to make galaxies behave as expected. These calculations are the same for most galaxies with dark matter, and galaxies without dark matter require no correction. At this point we have two explanations: either gravity behaves radically differently in different places in the universe, or there's a new type of massive particle that we haven't discovered. We know already that the standard model of particle physics is incomplete, and there are more particles to be discovered. We are also very confident that the laws of physics are the same throughout space and time, and we are very confident that our models of gravity are correct on large scales. The simplest explanation that fits all available data is dark matter. Positing that there's a new type of massive particle that we haven't discovered isn't really a shocking revelation.