There is a lot of evidence that motion and gravitational lensing at the scale of galaxies doesn’t behave how we would expect from the mass we see and relativity.

The obvious solutions there are:

1. There is mass we don’t see And/or
2. Our models of gravity aren’t fully correct

Dark matter is the theory with the most support, but there are plenty of problems with it as well. For one, we’d need the majority of the matter in the universe to be dark, and we have essentially no theory for what it would be. Obviously no way to ‘see’ it, and no way to detect it beyond gravitational effects. Any theory that relies on the presence of an absolutely massive amount of invisible and conveniently distributed stuff is going to face challenges.

MOND has also faced trouble, no theory here has accounted for all observations.

We must also remain open to the possibility that some observations are true evidence for an unknown massive particle, and some are the result of an incorrect understanding of gravity.