... The Matter of the Bullet Cluster . ... Explanation: The matter in galaxy cluster 1E 0657-56, fondly known as the "bullet cluster", is shown in this composite image . A mere 3.4 billion light-years away, the bullet cluster's individual galaxies are seen in the optical image data, but their total mass adds up to far less than the mass of the cluster's two clouds of hot x-ray emitting gas shown in red. ... Publications with keywords: dark matter - galaxy cluster . ... Grand Spiral Galaxy NGC 1232 . ...