For decades, astrophysicists have thought that some sort of mysterious dark matter must provide most of the gravity that glues galaxies together. For nearly as long, scientists have tried to spot the stuff interacting with ordinary matter in some other way—say, by looking for particles of it bouncing off atomic nuclei. Now, a team of astronomers reports a potential sign of dark matter interacting—although not with ordinary matter, but with itself. If it holds up, that interpretation would require a major rethink of astrophysics and cosmology. But more prosaic explanations are possible, others caution.
"We sat down and tried to think what else might explain [the observation]," says Richard Massey, an astronomer at Durham University in the United Kingdom and lead author on the new paper. "We haven't come up with anything yet."
So far, all the astrophysical and cosmological data suggest that dark matter interacts only through gravity. Studies of the afterglow of the big bang—the so-called cosmic microwave background—show that dark matter makes up 82% of all matter in the universe, meaning that only 18% of matter is the ordinary stuff that forms galaxies, stars, and planets. Enormous computer simulations show that, as the universe evolved, dark matter gathered into ever bigger clumps. The gravity of a clump or "halo" draws in the ordinary matter to form a galaxy within it.
Those simulations assume that dark matter doesn't interact through any force besides gravity, not even with itself. Ordinary matter, by contrast, interacts through other forces, including the electromagnetic force that binds electrons in atoms and produces light. As a result, hot gas exerts a pressure, so that when two clusters of galaxies collide they can blow the gas out of each other. Astronomers have exploited this difference to demonstrate, in the galaxy collisions, the existence of dark matter. In such collisions, the gas ends up separate from the dark matter, which can be traced by the way its gravity distorts the images of more distant galaxies.
In recent years, some theorists have suggested that dark matter might also interact with itself. Such self-interacting dark matter might help smooth over certain sticky points in the simulations, such as their tendency to produce too many small galaxies and galaxies whose densities peak too sharply in the center—the so-called core-cusp problem.
Now, Massey and 21 others report possible direct evidence of such self-interaction. Using NASA's orbiting Hubble Space Telescope and the European Southern Observatory's Very Large Telescope—a battery of four 8.2-meter telescopes on Cerro Paranal in Chile—they studied a cluster of galaxies known as Abell 3827, about 1.3 billion light-years away. Its core contains four old elliptical galaxies and lines up on the sky with a far more distant spiral galaxy like our Milky Way. Thanks to its gravity, the cluster acts like a lens, not only distorting the image of the more distant galaxy, but also making four different fun house images of it appear around the cluster.
By comparing and, with the aid of a computer, unscrambling those images, Massey and colleagues were able map out the mass in the cluster, as they explain online today in the Monthly Notices of the Royal Astronomical Society. And that's where the surprise arose: The dark matter halo for one of the galaxies in the cluster appears to be separated from the stars in the galaxy itself. That offset is the hint that dark matter may be interacting with itself, Massey says.
That's because when galaxies collide or interact, stars—in contrast to gas—should also interact only through gravity, as the distance between stars is so vast they never collide. But then, the stars and their dark matter halo should follow the same trajectories and remain united, Massey says. So the fact that the dark matter ends up separated from the stars suggests the dark matter interacts with itself in some way. "What you can take strongly is that we see an effect—that the dark matter does something different than the galaxy it once surrounded," he says.
"I can believe, potentially, the measured offset," says Maruša Bradač, an astronomer at the University of California, Davis. "But whether this is dark matter self-interacting or some [ordinary matter] effect, of which I can think of many, I think it's premature to say." For example, Bradač says, the churning interactions of the four galaxies themselves triggers the formation of stars separated from the dark matter halos. To test such possibilities, researchers need to model the detailed evolution of the cluster, she says. Massey says his team is already working on it.
Even if the observation is evidence that dark matter interacts with itself, that interaction appears to be about 1/1000 as strong as would be needed to explain away the core-cusp problem. In fact, 2 weeks ago Massey and colleagues published in Science a study of colliding galaxy clusters that limited the strength of such self-interactions. "That previous study showed that [the strength of the interaction] is low," Massey explains. The new result suggests that "it's really, really low, but it's not zero."