*This unattractive name refers to the fact that it was discovered in the Deep Lens Survey, is a cluster, and where it is located. Our team affectionately calls it the Musket Ball Cluster since it is similar to the Bullet Cluster yet older and slower.
Mapping the matter of the Musket Ball
Galaxy clusters are predominantly comprised of dark matter (~85% total cluster mass), which as the name implies emits no light. The remainder of the mass is in the form of baryonic matter (i.e. the periodic table stuff) the majority of which is diffuse intracluster gas (~14% total cluster mass), and galaxies (~1%).
Galaxies of the Musket Ball
Dark Matter of the Musket Ball
When two clusters collide their galaxies rarely collide with one another, due to there being so much space between the galaxies. Thus galaxies can be considered collisonless particles. The cluster gas on the other hand is dispersed somewhat evenly throughout each subcluster and as a results the gas of one subcluster is much more likely to collide with the gas of the other. The result being that much of the gas will stop at the point of collision while the galaxies pass right on through. As can be seen in the images above, the mass of the cluster largely overlaps the galaxies (white contours). Using this observation and knowledge that there is ~6 times more gas mass than galaxy mass, we can infer that dark matter comprises ~85% of the total cluster mass and is (nearly?) collisionless. In a sense illuminating the dark matter.
For a more detailed account please see our journal article "Discovery of a Dissociative Galaxy Cluster Merger with Large Physical Separation". Also check out the Chandra image release: DLSCL J0916.2+2951: Discovery of the Musket Ball Cluster.