There's a lot of different data out there and a lot of different theories. The dark matter hypothesis fits a lot of the data really well with a very simple model, few extra constraints or variables.

The alternative theories don't fit the entire set of data as well. Or they do, but by choosing additional complexity, rules, constants that are chosen to make the model work, but can't be explained otherwise.

Dark matter isn't a full explanation, no doubt, but imho it's the best we have.