Dark matter is… doing something strange.

The Guardian and physicist Jon Butterworth are looking at something unexpected that the Spitzer Space Telescope has noticed. Galaxies are put together in a way that could explain a few things about everything:

More than 13 years later, on 19 September 2016, an intriguing analysis of some of these observations was posted by three astrophysicists, Stacy McGaugh and Federico Lelli from Case Western reserve University, and Jim Schombery from the University of Oregon. The analysis seems to be telling us something surprising.

Firstly, the acceleration of those stars furthest from the centre of the galaxy is much higher than would be expected from gravitational attraction of the stars and gas alone. This is well-known and is one of the strongest pieces of evidence for the existence of Dark Matter. By including the gravitational attraction of a certain amount of Dark Matter, the results can be brought into agreement.

Secondly – and this is the surprising one – there seems to be a very simple relationship between the ‘expected’ acceleration (stars and gas only) and the observed acceleration. This is peculiar, because they have studied a wide range of different rotating galaxies. Some of them have a big bulge of stars in the middle, some don’t. Some have more gas than stars, some have more stars than gas. The Dark Matter fraction also varies between galaxies, and also within galaxies, as most are dominated by normal matter in the middle and by Dark Matter around the edges.

At very least this should be telling us something interesting about those interactions. At its most extreme, such an observation could be explained by saying there is no Dark Matter, but the gravitational force of the stars and gas is stronger than expected. That, however, would leave a huge number of other pieces of evidence for Dark Matter – for example in collisions between star clusters, where it doesn’t follow the normal matter – unexplained.

There is a quote attributed to the scientist and author Isaac Asimov:

The most exciting phrase to hear in science, the one that heralds the most discoveries, is not “Eureka!” (I found it!) but ‘That’s funny…’