[dandelany]

Alternatively:

The orbits around galaxies fail to follow the law of gravity of ordinary matter. The orbits are as if our gravity calculations, which seem to work well otherwise, are flawed.

Something in/near galaxies causes them to bend light as if gravity were different.

So, for that region, call it a flaw in the theory of gravity. Seems simple enough.

"Unseen, never-before-detected form of matter which doesn't interact with any other matter except gravitationally?" Maybe, but that seems to be the hard way to get an answer.

This is tongue-in-cheek of course, but it's hard to trust intuition in these realms, especially when one person's intuition is another person's "hard way" :)

[rcxdude]

Yeah, but AFAIK all the explanations based around adjusting our theory of gravity have failed to match the results, and tend to be pretty complicated and thus prone to overfitting.

[AnimalMuppet]

Whereas, since you can adjust the hypothetical distribution of dark matter to be whatever you need it to be to fit the data, you cannot falsify dark matter this way, since proving that no possible distribution of dark matter could fit the evidence is pretty much impossible.

Still, to me there seems to be some level of equal implausibility in saying "our theory of gravity is wrong" and in saying "our theory of matter is wrong".

[bzbarsky]

From my limited understanding, what's interesting about the dark matter approach is that if you pick the distribution of dark matter to fit the rotational velocity profiles of galaxies that also just happens to be the distribution that has a good fit to various other unrelated data sets (cosmic microwave background fluctuations, say).

Now maybe there's a deep underlying reason we don't know about that makes those unrelated data sets in fact related to each other. Or maybe it just happens that the dark matter actually exists. But the point is that the dark matter theories we have were absolutely falsifiable, as the article points out. They made predictions that were then tested and so far the predictions have been correct.

[AnimalMuppet]

I stand corrected.

[Retra]

We have much more experience with matter, because we can interact with it on a variety of scales in a lab. Gravity is much harder to interact with, and thus much harder to explore. That's why it is quite a bit more likely that our understanding of gravity is incorrect.