Galactic Pile-Up May Point to Mysterious New Dark Force in the Universe• Wired.com
The results come from observations of the Musket Ball Cluster, a vast celestial object located about 5.23 billion light-years away in the constellation Cancer. Galaxies are usually gravitationally bound to other galaxies, creating massive galactic clusters. The Musket Ball Cluster is an example of what happens when two such galactic clusters – each composed of hundreds of individual galaxies – crash into one another.
Scientists know the visible stars in these galaxies make up only about 2 percent of the total mass in the cluster. About 12 percent of the mass is found in hot gas, which shines in X-ray wavelengths, while the remaining roughly 86 percent is made of invisible dark matter.