[Jevon23]

There’s about as much evidence for the existence of dark matter as we can possibly get, short of actually observing it.

In particular we’ve observed some galaxies that seem to have little to no dark matter. Gravitationally, they behave the way that you would “naively” predict without the need to plug in a correction term that represents dark matter. This indicates that there is something physically different about these galaxies (like an unobserved substance), and it points away from MOND-style theories.

[lottin]

You're saying that the evidence for the existence of dark matter is that your theory doesn't agree with the observation. This is like saying that you have no evidence at all.

[exe34]

No, we're saying we haven't come up with any alternative that's as good at explaining a whole lot of things.

For what it's worth, I hate it too, and I'm quietly betting on the paper a while back that showed GR was all you need as long as you can be bothered to do the really really hard maths.

[Sharlin]

Nope. It's saying that the theory didn't agree with the observation, so we came up with a hypothesis, and – guess what – we haven't found any evidence against that hypothesis, and a lot of evidence for that hypothesis even in places we didn't think we'd find some! That's 100% proper scientific method. Please don't argue against stuff you only have a strawman understanding of. Even if you're on HN and think you're smart and all.

[lottin]

What evidence against the existence of dark matter should we expect to find if it didn't exist?

[Sharlin]

That's easy. We look at the predictions a DM model makes, and then see if the predictions match observations. People seem to think that just because DM does not interact electromagnetically, the DM hypothesis cannot make testable predictions.

Models are quantitative entities – this is physics we're talking about. They almost always have quantitative parameters whose values initially have large uncertainties. We make observations in order to tighten the error bars of those parameters – to rule out parameter values inconsistent with observations.

Now, if we found that two observations are inconsistent with each other – each implies a parameterization incompatible with the other – and could rule out experimental error via repeat experiments etc, then that would be evidence against the entire model. In dark matter's case, we might realize, for example, that we cannot predict both galactic rotation curves and CMB anisotropies with a single DM model. That would be evidence against DM. (But that's not the case, both seem to be perfectly consistent with a single, well-constrained model.)