The nature of dark matter, the mysterious stuff whose gravity presumably binds a galaxy together, is sinking back into the shadows. Three years ago, a team of astronomers reported that dark matter might interact with itself through some force other than gravity, a hint that might help theorists figure out what it is. But new observations rule out such interactions, the same team reports today at the joint European Week of Astronomy and Space Science and National Astronomy Meeting 2018 in Liverpool, U.K.
Previously, researchers using NASA's orbiting Hubble Space Telescope studied a cluster of galaxies 1.3 billion light-years from Earth called Abell 3827, whose gravity distorts and multiplies the image of a more distant galaxy. From the distortions, researchers deduced the distribution of dark matter in the cluster. One of the four galaxies in its center appeared separated from the clump or halo of dark matter that ought to envelop it. Modeling suggested that the two became separated because the dark matter interacted with itself. Any such interaction might help theorists figure out what sort of new particles constitute dark matter—something that's hard to do if all they know is that dark matter is massive stuff that produces gravity.
But new observations with the Atacama Large Millimeter/submillimeter Array, an array of radio dishes in the Atacama Desert in Chile, reveal that the galaxy hasn't separated from its halo after all. The data are now consistent with the less-than-helpful idea that dark matter interacts only through gravity.