<p>X-ray photo by <a href="https://en.wikipedia.org/wiki/Chandra_X-ray_Observatory" class="extiw" title="w:Chandra X-ray Observatory">Chandra X-ray Observatory</a> of the <a href="https://en.wikipedia.org/wiki/Bullet_Cluster" class="extiw" title="w:Bullet Cluster">Bullet Cluster</a> (1E0657-56). Exposure time was 0.5 million seconds (~140 hours) and the scale is shown in <a href="https://en.wikipedia.org/wiki/megaparsecs" class="extiw" title="w:megaparsecs">megaparsecs</a>. Redshift (<i>z</i>) = 0.3, meaning its light has wavelengths stretched by a factor of 1.3. Based on today's theories this shows the cluster to be about 4 billion light years away.
</p><p>In this photograph, a rapidly moving galaxy cluster with a shock wave trailing behind it seems to have hit another cluster at high speed. The gases collide, and gravitational fields of the stars and galaxies interact. When the galaxies collided, based on black-body temperture readings, the temperature reached 160 million degrees and X-rays were emitted in great intensity, claiming title of the hottest known galactic cluster.
</p><p>Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of <a href="https://en.wikipedia.org/wiki/dark_matter" class="extiw" title="w:dark matter">dark matter</a>.
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Sister picture: <a href="//commons.wikimedia.org/wiki/File:Bullet_cluster_lensing.jpg" title="File:Bullet cluster lensing.jpg">image:bullet_cluster_lensing.jpg</a>
<a href="//commons.wikimedia.org/wiki/User:Mac_Davis" title="User:Mac Davis">User:Mac_Davis</a>
<a rel="nofollow" class="external free" href="http://cxc.harvard.edu/symposium_2005/proceedings/files/markevitch_maxim.pdf">http://cxc.harvard.edu/symposium_2005/proceedings/files/markevitch_maxim.pdf</a>