And that's why there's groups working on both dark matter and modified/new theories of gravity.

It seems quite difficult to modify GR or make a new theory of gravity which explains the new observations while matching GR in the old ones, where GR has been verified. But if someone manages, then you'll see scientists flock to it.

But the hypothesis of dark matter fits all of the evidence, while no alternative theory of gravity comes close. That's what makes dark matter so attractive to physicists.

I'm not shunning the idea, I've worked on alternative explanations myself (with no concrete result), but for the moment we remain convinced that dark matter is the better theory.