Dark heart of the Musket Ball cluster (Image: X-ray: NASA/CXC/UCDavis/W.Dawson et al; Optical: NASA/STScI/UCDavis/W.Dawson et al.)

Colliding clusters of galaxies may hold clues to a mysterious dark force at work in the universe. This force would act only on invisible dark matter, the enigmatic stuff that makes up 86 per cent of the mass in the universe.

Dark matter famously refuses to interact with ordinary matter except via gravity, so theorists had assumed that its particles would be just as aloof with each other. But new observations suggest that dark matter interacts significantly with itself, while leaving regular matter out of the conversation.

“There could be a whole class of dark particles that don’t interact with normal matter but do interact with themselves,” says James Bullock of the University of California, Irvine. “Dark matter could be doing all sorts of interesting things, and we’d never know.”


Some of the best evidence for dark matter’s existence came from the Bullet cluster, a smash-up in which a small galaxy cluster plunged through a larger one about 100 million years ago. Separated by hundreds of light years, the individual galaxies sailed right past each other, and the two clusters parted ways. But intergalactic gas collided and pooled on the trailing ends of each cluster.

Mass maps of the Bullet cluster showed that dark matter stayed in line with the galaxies instead of pooling with the gas, proving that it can separate from ordinary matter. This also hinted that dark matter wasn’t interacting with itself, and was affected by gravity alone.

Musket shot

Last year William Dawson of the University of California, Davis, and colleagues found an older set of clusters seen about 700 million years after their collision. Nicknamed the Musket Ball cluster, this smash-up told a different tale. When Dawson’s team analysed the concentration of matter in the Musket Ball, they found that galaxies are separated from dark matter by about 19,000 light years.

“The galaxies outrun the dark matter. That’s what creates the offset,” Dawson said. “This is fitting that picture of self-interacting dark matter.” If dark matter particles do interact, perhaps via a dark force, they would slow down like the gas.

This new picture could solve some outstanding mysteries in cosmology, Dawson said this week during a meeting of the American Astronomical Society in Long Beach, California. Non-interacting dark matter should sink to the cores of star clusters and dwarf galaxies, but observations show that it is more evenly distributed. If it interacts with itself, it could puff up and spread outward like a gas.

So why doesn’t the Bullet cluster show the same separation between dark matter and galaxies? Dawson thinks it’s a question of age – dark matter in the younger Bullet simply hasn’t had time to separate.

New window

The idea complements a previous study that saw evidence for dark forces at work in the Bullet cluster. In 2007, Glennys Farrar of New York University and colleagues said that the smaller cluster was moving too fast for gravity alone to be responsible. They suggested that some mysterious force related to dark matter might be hurrying it along.

Still, two clusters is not a lot to go on. Dawson, Bullock and colleagues are following up with about 20 more galactic collisions to see if they show any unusual behaviour. “I really think that we’re almost to the point where we have enough observational data in hand,” Dawson said. “We could close the book on self-interacting dark matter.”

If the new force does exist, we might soon be able to see its effects on things influenced by dark matter, such as the behaviour of black holes or the masses of the first stars, says Douglas Finkbeiner of Harvard University, who was not involved in the new study.

“The simple thing isn’t always the right thing, so I really appreciate that Will is trying to look into these other possibilities,” he says.

Louis Strigari of Stanford University in California agrees. “Self-interacting dark matter is worth pursuing because we’re still very ignorant,” he said. “We’re desperate to understand what dark matter is, so any new window is welcome.”