Colliding galaxies
A composite image of two colliding galaxy clusters showing x-ray and optical radiation. Hot gas detected by Chandra in x-rays is seen as two pink clumps in the image and contains most of the "normal," or baryonic, matter in the two clusters. An optical image from Magellan and the Hubble Space Telescope shows the galaxies in orange and white. The blue areas in this image show where astronomers find most of the mass in the clusters. Most of the matter in the clusters (blue) is clearly separate from the normal matter (pink), giving direct evidence that nearly all of the matter in the clusters is dark.
(Image courtesy of Chandra X-ray Observatory.)