> I don't understand why consensus jumped to the conclusion that there must be a new form of undetectable matter instead trying to figure out what we don't know about gravity.

Is it implied in "I don't understand" that "it's not understandable?" There is quite a bit of evidence that point towards dark matter and away from problems in our theories of gravity. We have direct evidence for forms of matter that do not interact with particular fields, it would be unsurprising to find other forms that are extremely difficult or impossible to detect.