As an alternative, could we also be wrong about the gravitational constant at a large scale, or perhaps it's not really exactly an inverse square? Or maybe there's another relatively weak force that's only apparent at a larger scale that maybe only drops off linearly so it's predominant at large distances?

The Google term for that if you want to read more about it is "MOND", MOdified Newtonian Dynamics. It's not the theory favored by the consensus, but it's not out of the fight yet.

There's also TEVES which is MOND with relativity, since MOND doesn't predict any of the relativistic effects we observe constantly.

The problem is, they still end up needing some dark matter before they work very well. I think every bright person assumes dark matter must be fake when they hear about it, but nobody has succeeded in getting rid of it. Dark matter plus the physics we know about is actually a really great fit to what astronomers see, and it has been hard to improve on.

https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...

Yes, I should say that I'm not necessarily advocating for it, just giving a hint as to where to look the ideas up.

Personally I'm just fine with either. Particle physics makes it pretty clear IMHO that the idea of a weakly interacting massive particle is perfectly reasonable, because the particle zoo that we have is already rich far beyond "normal matter" and adding a few more isn't conceptually too difficult. (It'll break our theories, of course, but we kinda already know there must be something wrong with them, so that's not a huge shocker.) And the idea that physical laws may not be exactly perfectly the math we thought they were is also not that shocking to me, for all kinds of reasons, again not least of which is that we know something is wrong with our current theories.

Awesome. Thanks for the starting off point!