[lisper]

"Dark matter" is a synonym for "some unknown stuff that causes experiment to disagree with theory based on what we currently observe." It's not dark matter per se that contradicts GR, it is the experimental observations that require dark matter to be postulated in the first place that contradict GR.

[munchler]

No, there's nothing in the dark matter hypothesis or observations that necessarily contradicts GR.

However, it's true that some other theories that claim to explain the dark matter observations are in conflict with GR, which is why those theories are received very skeptically.

[lisper]

> there's nothing in the dark matter hypothesis or observations that necessarily contradicts GR

That is true for the "hypothesis" part, it is not true for the "observations" part. The observations do contradict GR. To be precise, they are at odds with the predictions of GR on the assumption that the universe is made entirely of ordinary matter. So there are two possibilities: 1) GR is wrong, 2) the universe is not made entirely of ordinary matter, i.e. there is "something else", which we call "dark matter". The problem is, there is no evidence for the existence of dark matter other than the observations that are at odds with the predictions of GR, and so the possibility that GR could be wrong still needs to be taken seriously at this point.

[matthewdgreen]

“The observations do contradict GR” and “the observations are at odds with GR under the assumption that the universe is made of ordinary matter” are two very different statements, and the latter does not imply the former.

You could also say that deviations in Uranus’s orbit contradict GR under the assumption that Neptune doesn’t exist. But Neptune does exist, so this isn’t really a statement about GR at all.

[lisper]

The difference is that Neptune is there, indeed, was discovered because it was exactly where Newtonian mechanics predicted it would be. The situation here is completely different because all of the plausible hypotheses about the nature of dark matter have been tested and falsified. The only thing left is "some weird stuff that is fundamentally unlike anything we have ever observed before". It is more analogous to the luminiferous aether than to Neptune. If that doesn't count as contradicting GR, at least potentially, then nothing contradicts anything because you can always resort to this kind of special pleading to explain any observation under any hypothesis.

[oh_my_goodness]

Are you suggesting that all of the plausible hypotheses about the nature of dark matter have been tested and falsified?

[lisper]

I'm not suggesting it, those were literally the exact words that I wrote.

Of course, that claim turns entirely on the meaning of "plausible", so I'll amend my claim to simply say that all attempts to detect dark matter to date have failed.

[User23]

Which is odd, because by far our best experimentally verified theory is also in conflict with GR. Our measurements for QED are orders of magnitude more accurate, so I'm inclined to believe GR is the incorrect theory.

[adgjlsfhk1]

Some of our measures of QED are very accurate (10 decimal places give or take), but the measurements QCD are much more complicated and the theory doesn't produce nearly as clean values. Also, there are purely QED experiments (muon G2 for example) where there are some serious contradictions.

[oh_my_goodness]

Are you saying that GR has been contradicted by experimental observations?