What a cluster: When dark matter collides, things get weird

Dark matter just got a shade more mysterious. Scientists studying a smashup between giant clusters of galaxies have watched how each cluster’s dark matter passes through the collision – and found that it seems to contradict certain theories of how dark matter behaves.

The findings, published in the journal Science, show that further study of these dramatic collisions could shed light on a very dimly understood part of the cosmos. But for the moment, they leave wide open the question of what dark matter is.

Dark matter can’t be touched or seen, but it makes up 84.1% of all the matter in the universe. That means that for every one share of normal matter – the stuff that makes up the stars, galaxies, black holes, our planet, every proton and elctron and other subatomic particle – there are more than five shares of this unknown stuff called dark matter.

And yet, in spite of its overwhelming presence, scientists have little idea what dark matter is. It makes its presence known through its gravitational influence: Dark matter accounts for the vast majority of stuff in galaxies and galaxy clusters, and all that extra mass dramatically speeds up the spin of galaxies and helps define the structure of the cosmic web.

There are a number of theories as to what dark matter might be. Some think it is a weakly interacting massive particle, or WIMP, which almost never interacts with normal matter. (Scientists have set up dark-matter detectors to search for these WIMPS, on the chance that they might – on very rare occasions – interact with a normal matter particle.)

Others think dark matter might be “self-interacting,” which means it could interact with other dark matter particles. Dark matter may represent a “hidden sector” with a whole menagerie of different types of particles, separate from normal matter, or it could be “mirror matter,” and have alternate versions of all the particles in normal matter – a “dark neutron,” for example.

Still others think that dark matter may not be a particle at all, but perhaps a misunderstanding of the rules of gravity, or perhaps some kind of quantum scarring left by the universe’s violent birth.

Researchers study the dark matter in galaxies and galaxy clusters, but studying what happens when two enormous clusters crash into each other offers a unique opportunity to examine dark matter.

“Galaxy clusters that are merging with each other represent interesting laboratories for this kind of question,” said James Bullock, a UC Irvine astrophysicist who was not involved in the study.

Galaxy clusters are already about 90% dark matter, and so if dark matter does indeed interact strongly with itself, these smashups are the right place to see whether anything happens. The scientists used data from NASA’s Hubble Space Telescope and Chandra X-ray Space Observatory to study galaxy cluster mergers, particularly the “bullet cluster” collision, a projectile-shaped structure resulting from two clusters ramming into each other.

When it comes to normal matter, galaxy clusters are mostly empty space, so the likelihood that individual stars would smash into one another is fairly low. The clusters’ stars, planets, galaxies — their normal matter — can whiz right past one another as the clusters collide. But when it comes to dark matter, there is so much of it that each cluster’s dark matter will probably cross paths — and if the particles do interact, the effects should show up in in the smashup’s structure.

“If dark matter’s particle interactions are frequent but exchange little momentum … the dark matter will be decelerated by an additional drag force,” the study authors wrote. “If the interactions are rare but exchange a lot of momentum … dark matter will tend to be scattered away and lost.”

So if dark matter interacts with itself a lot, it will experience some “drag” – kind of like trying to make your way through a crowded city street. The normal matter, on the other hand, will have a clear and easy path forward because there’s far less of it in each cluster. If this is the case, then as the dark matter from one cluster is flying through the dark matter of the other cluster, it shouldn’t be able to keep up with the normal matter.

So the scientists looked to see whether there was a discrepancy in the positions of the normal matter and the dark matter. But they found that there wasn’t much lag between the normal matter and the dark matter – the dark matter was keeping pace just fine, which means it probably wasn’t getting slowed down by interacting with other dark matter particles.

The findings could help narrow the possibilities for what form dark matter takes, the study authors said.

But at the moment, scientists may not know enough about the dynamics within these clusters to make accurate predictions of what dark matter must be like, Bullock said.

“I like the idea of using these clusters this way, and I think that the observations are excellent,” Bullock said. But he added, “It’s hard to know exactly what’s going to happen when you have dark matter that’s complex like this.”

Better and more sophisticated computer modeling could help address these questions, he said.

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