The most useless law in nature ?

In which I summarise the current debate over whether this apparent "new law of nature" means anything or not, and conclude (spoiler !) that it probably doesn't.

It's been known for many years that there's a correlation between how fast matter in a galaxy rotates and how much gas and stars it contains. The problem is that there shouldn't be a nice relation, because galaxies seem to be dominated by dark matter. This strange relationship between normal and dark matter, which should be independent of each other, has been shown in different ways over the years. Recently it was claimed that all of these are just manifestations of a deeper underlying "law" : the Mass Discrepancy Acceleration Relation (or technically the Radial Acceleration Relation, but whatever).

The problem is that the expected acceleration of matter (based on its mass and standard Newtonian gravity) correlates very, very well with its actual acceleration. If there is, as we think for many other reasons, actually a huge amount of unseen "dark matter" present, then it shouldn't do that. Could it be that the dark matter theory is wrong ?

YES ! But it probably isn't. Although modified theories of gravity do predict this relation - indeed, did predict it 30 years ago - it seems that it also occurs in standard dark matter simulations without any difficulties. You can see a comparison of observations (left) and the simulations (right) in the example image below. Since both modifying gravity and using dark matter give the same result, this discovery is probably useless. Weird, but useless.

Read on for tales of academics behaving badly, pretty pictures of galaxies, lots of and lots of graphs, and a surprised dog...

Placeholder post intended to be replaced with a slightly better summary.