When two giant galaxy clusters collided, a collision was observed so strong that their dark matter essentially separated from normal matter and flew away.

It is like a collision between two vehicles with untied cargo. The vehicles themselves are stopped by other forces, but the cargo continues to move under the influence of dynamics.

It’s not the first time we’ve seen dark matter uncoupling from normal matter in the course of a giant cosmic collision, but we’ve never seen it like this before. And the discovery, scientists hope, could give us new clues about the behavior and properties of the mysterious dark matter that dominates the universe.

Dark matter is one of the great mysteries of the universe. We neither know what it is nor can we detect it directly.

But somehow there is more gravity out there in the cosmos than we would expect from the “normal” matter we can measure. Much more gravity. We observe it in the motions of stars and galaxies and in the way spacetime itself bends and deforms in the presence of strong gravitational fields.

Accordingly scientist Calculations suggest that only about 15 percent of the universe’s matter budget is made up of normal matter. The remaining 85 percent or so is this invisible, untouchable dark matter, which only seems to interact with normal matter through gravity.

And it’s everywhere. Normal matter galaxies like our own exist in huge clumps of dark matter. Huge strands of it form what we call the cosmic web, which gravitationally connects galaxies to one another, galaxy clusters to galaxy clusters, and provides pathways for normal matter to meet, collide, and grow.

Normal matter interacts with other things in many other ways, so massive events – like collisions between galaxy clusters – can get interesting. By and large, the galaxies inside the clusters are fine, but in intragalactic space there are huge clouds of gas colliding with each other, getting hot and turbulent.

Since this normal matter interacts electromagneticallyand when they are subjected to shocks and turbulence, a braking effect is triggered, slowing the clusters down. However, the dark matter remains unaffected by these braking influences and continues to move before eventually settling back into its position around the newly merged cluster.

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The merging clusters are collectively called MACS J0018.5+1626 and are aligned in a way that allows a new measurement of the speed of matter within them.

An earlier measurement was made on a cluster collision that we are observing from the side; MACS J0018.5+1626 is oriented so that one cluster appears to be coming directly toward us and the other directly away from us.

The researchers used different techniques to measure the velocities of dark and normal matter.

The speed of dark matter was determined by measuring the speed of galaxies within galaxy clusters, determining how light is stretched or compressed towards red or blue as it moves away from us or towards us. This is called the Doppler shift and is a reliable way to measure speed in space.

The speed of the galaxies belonging to this cluster serves as an indicator of the speed of dark matter, since galaxies and dark matter behave similarly in a cluster collision.

To measure the speed of the intracluster medium, the team used what is known as the Sunyaev-Zeldovich effect, a significant distortion of the background light of the universe caused by electrons in a volume of space. Since these electrons can scatter light, this effect can be used to determine how densely packed the electrons are.

A secondary effect can be observed when the scattering medium is moving, and scientists can analyze this effect to determine how the medium is moving: how fast and in what direction.

This allowed the researchers to determine the speed of the normal matter in the clusters and observe a clear difference in speed between this and the dark matter visible in the galaxies.

It is a very thorough work of astrophysics that could help us in our attempt to understand exactly what dark matter is.

“This study is a starting point for more detailed studies of the nature of dark matter,” says Silich. “We have a new type of direct investigation that shows how dark matter behaves differently than normal matter.”

The study was published in The Astrophysical Journal.