A freshly discovered system of colliding galaxy clusters has been nicknamed the "Musket Ball Cluster", because it's a much older and slower version of the previously-discovered "Bullet Cluster".

The Bullet Cluster (aka 1E 0657-558) is a famous collision between two galaxy clusters, because it gave astronomers the best evidence to date for the existence of dark matter (a mysterious and invisible type of matter that constitutes some 83 percent of all matter in the universe).

Here's how it happened. As the two clusters of galaxies crisscrossed, the distant stars passed by each other unscathed. But the surrounding gas clouds violently clashed, and they bumped and rubbed into each other.

The ensuing friction slowed the clouds down, but the stars kept up their pace and whizzed through the cosmos. This left that the gas clouds in stasis, and the galaxies slipped out into clear space.

This gave astronomers a chance to estimate the total mass and gravity of the galaxy clusters without all that hot gas. But after doing the maths, they found that the the galaxies lit up far too much for their size.

It meant that an unseen force had escaped the clouds along with the galaxies and was helping to bend more light to the Chandra X-ray Observatory. For the first time, astronomers caught dark matter -- red handed.

Back in 2006, when the Bullet Cluster was discovered, Nasa wrote, "a team of scientists has found direct evidence that dark matter is as real as the rings around Saturn."

There have been five other examples of clusters that merged in such a way that normal and dark matter was ripped apart. But in these six systems, the collision is estimated to have occurred between 170 million and 250 million years ago.

Observations of the Musket Ball Cluster, however, come about 700 million years after the original collision. Nasa's Chandra X-ray Observatory and Hubble Space Telescope as well as the Keck, Subaru and Kitt Peak Mayall telescopes, were used for observation.

The prehistoric collision will give astronomers valuable insight into how galaxy clusters grow and change after major collisions. Plus, the Musket Ball Cluster will show whether dark matter can interact with itself. This information is important for narrowing down the type of particle that may be responsible for that mystery matter.


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