(via Cosmic Log - Dark matter blob confounds experts)
Click through for the run-down, but this is an overlay of HST data from the Wide Field 2 camera of the Abell 520 galaxy cluster. The visible matter is in orange. It’s known that the cluster has gone through a high-speed collision recently, but what’s weird is that the dark matter (in blue) and the hot gasses (in green) have trailed in the collision.
Why is that weird? Well, this is the second observation of a known high-speed collision result, the first being the Bullet cluster, show here with visible and Chandra x-ray data merged:
(Image courtesy SLAC National Accelerator Laboratory, Stanford.)
Note that the blue dark matter leads the visible matter (in red) in this collision result.
Many astrophysicists were hoping that the Abell 520 observations would confirm their theories about how dark matter acts in the universe. However, as the results seem to be different, the team that did the Abell 520 observations are going to run through a whole mess of simulations to try to find out if there’s a configuration of the visible matter and expected properties of dark matter that would allow this different result. Alternatively, the particle physicists might have to figure out a way to find more than one type of dark matter, one that behaves as in the Bullet collision and one that is more “sticky” like in the Abell 520 event.
There’s a nice simulation of the original Bullet Cluster collision on the SLAC page here:
(Sources: HST, MSNBC.com, SLAC, Chandra)