"For astronomers studying dark matter, the Bullet Cluster is one of the greatest laboratories in the universe. It was discovered almost by accident, a blip of x-rays in the sky that was detected by NASA's Einstein Observatory in 1992 and given the designation 1E 0657-56. Follow-up observations in visible light confirmed it to be a galaxy clustera swarm of dozens or even hundreds of galaxies all bound together by gravity and orbiting a common center."
"The 1E 0657-56 cluster is decently far away from Earth; the light we see left it about four billion years ago. The cluster is not just one simple system of galaxies, though. There's a main cluster, big and somewhat elongated, with a more compact and spherical subcluster offset to one side, separated by more than 1.5 million light-years. Note that the nearest big galaxy to our own Milky Way is the Andromeda galaxy, 2.5 million light-years away."
1E 0657-56, known as the Bullet Cluster, contains a large main galaxy cluster and a compact spherical subcluster separated by over 1.5 million light-years. The system lies about four billion light-years away. X-ray observations reveal the cluster contains vast amounts of intracluster gas at tens of millions of degrees Celsius. Such gas commonly becomes superheated by radiation from supermassive black holes and by kinetic energy from orbiting galaxies. The hot gas in this system is divided into two main clouds located between the main cluster and the subcluster, with one cloud elongated and diffuse and the other smaller and closer to the subcluster.
Read at www.scientificamerican.com
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