Photo: El Gordo (ACT-CL J0102-4915): NASA's Chandra Finds Largest Galaxy Cluster in Early Universe
http://chandra.harvard.edu/photo/2012/elgordo/

This galaxy cluster, which has been nicknamed "El Gordo" for the "big" or "fat" one in Spanish, is a remarkable object. Found in the distant Universe by Chandra and the Atacama Cosmology Telescope, El Gordo appears to be the most massive, the hottest, and gives off the most X-rays of any known cluster at its distance or beyond. In this composite image of El Gordo, X-rays are blue, optical data from the Very Large Telescope are red, green, and blue, and infrared emission from Spitzer is red. The comet-like shape of the X-rays, along with optical data, show that El Gordo is actually the site of a collision between two galaxy clusters, similar to the well-known Bullet Cluster.
Photo: Abell 520: Dark Matter and Galaxies Part Ways in Collision between Hefty Galaxy Clusters

http://chandra.harvard.edu/photo/2012/a520/

This composite image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520. Data from Chandra (green) show the hot gas in the clusters and provides evidence that a collision took place. Optical data from Hubble and the Canada-France-Hawaii Telescope in Hawaii are shown in red, green, and blue. Starlight from galaxies within the clusters, which has been smoothed to show the location of most of the galaxies, is colored orange. Confirming a previous observation, this result reveals that a clump of dark matter resides near most of the hot gas, where very few galaxies are found.
Photo: Abell 383: Getting a Full Picture of an Elusive Subject
http://chandra.harvard.edu/photo/2012/a383/

Two teams of astronomers have used data from Chandra and other telescopes to map the distribution of dark matter in three dimensions in the galaxy cluster Abell 383. The dark matter in Abell 383 is stretched out like a gigantic football with the point of the football aligned close to the line of sight. The X-ray data (purple) from Chandra in the composite image show the hot gas, which is by far the dominant type of normal matter in the cluster. Galaxies are shown with the optical data from the Hubble, the Very Large Telescope, and the Sloan Digital Sky Survey, colored in blue and white.
Video: El Gordo (ACT J0102-4915) in 60 Seconds
Astronomers using the Chandra X-ray Observatory and ground-based optical telescopes have discovered an extraordinary galaxy cluster some 7.2 billion light years from Earth.

http://www.chandra.harvard.edu/photo/2012/elgordo/

http://chandra.si.edu/resources/podcasts/sd.html
Video: Abell 383 in 60 Seconds!

Dark matter is mysterious. We know that it is invisible material that does not emit or absorb any type of light, but we can detect it through the gravitational effects it has on material we can see.

http://www.chandra.harvard.edu/photo/2012/a383/

http://chandra.harvard.edu/resources/podcasts/sd.html
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Abell 383: Getting a Full Picture of an Elusive Subject
http://chandra.harvard.edu/photo/2012/a383/

Two teams of astronomers have used data from Chandra and other telescopes to map the distribution of dark matter in three dimensions in the galaxy cluster Abell 383. The dark matter in Abell 383 is stretched out like a gigantic football with the point of the football aligned close to the line of sight. The X-ray data (purple) from Chandra in the composite image show the hot gas, which is by far the dominant type of normal matter in the cluster. Galaxies are shown with the optical data from the Hubble, the Very Large Telescope, and the Sloan Digital Sky Survey, colored in blue and white.

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