This just released image shows an X-ray view from the orbiting Chandra X-ray observatory overlaid on a visible light image of galaxy cluster Abell 520 The optical part shows stars in our Milky Way and galaxies belonging to the cluster. The red glow is the x-rays from normal matter heated to millions of degrees in the cluster. The blue denotes the location of the dark matter in the cluster, inferred by its distortion of the coordinates of background galaxies.
Paradoxically, more dark matter appears near the center of the cluster, where galaxies are few , while some areas where there are lots of galaxies show little dark matter.
This is antithetic to what theory predicts- astronomers are baffled that the collision of the Abell 520 cluster tore galaxies away from both dark
matter (blue) and the x-ray emitting hot gas clouds (magenta). The circles show where
astronomers had expected the galaxies to end up.
Credit: NASA/CXC/M. Weiss