I believe the rotational speed discrepancy in galaxies is simply the most intuitive example. There are many more cases where mass appears to be missing: https:/

I believe the rotational speed discrepancy in galaxies is simply the most intuitive example. There are many more cases where mass appears to be missing: https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... . Crucially, these occur at vastly different scales, from individual galaxies to clusters and whole observable universe. > ...have we really jumped to matter being the culprit without taking a look at gravity behaving in unexpected ways? This is called Modified Gravity, or MOND: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics and is well-researched. It stands as a niche but actively pursued alternative to Dark Matter. For the most part, Dark Matter fits observations better and more parsimoniously. But of course we don't have definitive proof/disproof for either.