Galactic Pile-Up May Point to Mysterious New Dark Force in the Universe

LONG BEACH, California — By closely mapping the mass of an enormous galactic collision, astronomers may have uncovered a type of force that only affects dark matter.

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. Because the galaxies make up so little of the mass of the system and the spaces between them are so large, they don’t really do much of the crashing. Odds are that they will simply sail by one another as the clusters merge. It’s mostly the gas that collides, causing it to slow down and fall behind the galaxies.

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