Thanks. I'm also no expert - I'm just learning general relativity - but that's also my rough understanding: either there needs to be a modification of the theor

Thanks. I'm also no expert - I'm just learning general relativity - but that's also my rough understanding: either there needs to be a modification of the theory, or there's a new form of matter. It might seem more parsimonious to modify the theory, but then how do you do that in a way that retains all the successful predictions of GR while explaining the recalcitrant observations? That's the hard part. It seems at the moment that the minimal and most elegant adjustment to the worldview required is to postulate the new form of matter. But I think it's safe to say it's a genuine problem in our knowledge: we don't know how to solve it