As mentioned elsewhere, I'm an idiot not a physicist, but as I understand it dark matter is favored over modified gravity theories because all such theories are

As mentioned elsewhere, I'm an idiot not a physicist, but as I understand it dark matter is favored over modified gravity theories because all such theories are considered to be lacking in parsimony. Essentially, instead of showing how the behavior we observe arises from a fundamental simplicity, they are adding and tuning parameters to fit. With enough parameters one can fit any arbitrary shape, but this doesn't give the model any predictive power. I also get the impression that while dark matter is popularly accepted as "actually existing", that is much less the case for dark energy.