Go freaking figure, just a day after ranting on about RHIC physics to a whole group of friends who trekked out to BNL from NYC, those damn astronomers have got to get some cheap media attention by doing something easy: like finding dark matter. What I find somehow pleasing, though, is how conceptually familiar it all feels. Hereby begins a novice's description
The image shows a collision of two galaxy clusters, but imaged in three different ways. The optical image shows flashes of white galaxies, something we are used to seeing through telescopes. The pink shows an image of the distribution of hot gas surrounding the cluster, imaged by X-ray astronomy (i.e. by looking at the photons emitted by the gas), which seems to have taken on an oddly "Mach Cone" shape, if I may add. Finally the bluish clumps are the distribution of mass, as imaged by the distorted views of the galaxies behind (i.e. gravitational lensing).
The rub lies in the way the pink and the blue have separated. The general view is that the gas has slowed down as the clusters passed through each other, while the dark matter has continued moving on. But if the gas was somehow the more massive part of the system, one would have also found the mass there as well.
The familiar part is that my colleagues and I have been pondering what happens when clumps of things we like, strongly-interacting nucleons (i.e. nuclei), pass through each other at high speed. We also try and "image" the space time story, but of course using different tools. We can't see spatial distributions directly, but we can infer them from the final velocities. Of course, they can't repeat their collisions, while we do so thousands of times per second. And we're all using theories, like hydrodynamics, which are amazingly applicable at extremely microscopic and extremely macroscopic scales.