Dark Matter Is Dark Even to Other Dark Matter

​Dark matter's claim to fame is that it doesn't interact very well with regular matter. Thanks to the electromagnetic force, we here in the world of normal matter get to experience photons and electrons as light and electricity, but dark matter misses out. This is the whole problem with directly detecting the stuff, which would seem to mostly just interact via gravity. As such, dark matter might be viewed as a ghost universe unto itself, mingling but never "touching."

New observations courtesy of NASA's Hubble Space Telescope and Chandra X-ray Observatory offer an interesting twist to our (typical) conceptualization of dark matter. It seems that it doesn't even interact with itself, at least in terms of exchanging momentum. This is to say that if you fire a dark matter cannonball at a dark matter wall, the ball will just sail on through. It's not the most unexpected thing, but it allows physicists to now add new additional constraints to dark matter models, offering a clearer picture of what the stuff even is (or should be).

The new NASA results are published in the current issue of Science. They're based on observations of colliding galactic clusters, where enormous amounts of dark matter are found mingling with individual galaxies, stars, and huge clouds of gas. These clusters offer a natural laboratory for astronomers to study interactions between dark matter and regular matter via gravitational lensing, which is the main way that dark matter makes itself visible.

"Specifically, what these collisions reveal is dark matter's interaction "cross-section," which is a measurement of attenuation, or how much resistance a material puts up against another material (how easily the cannonball travels through the wall). The astronomers were able to come up with a self-interaction cross-section for galactic dark matter by tracing the behavior of 72 different galactic clusters, which are visible to Hubble and Chandra from many different angles and at different times.

The basic idea is that as a galaxy cluster collides with another galaxy cluster, their respective constituent stars shouldn't slow down very much as gas clouds offer low resistance to stars, while the stars themselves are spaced so far apart that collisions would be rare enough to not offer much of a barrier. (The star component of galactic clusters would have a very small interaction cross-section.) Meanwhile, dueling colliding gas clouds would interact, basically grinding to a halt (a very large interaction cross-section).

As two galaxy clusters collide, the effect is that stars sail on through while gas stops. This makes it possible to gauge dark matter interactivity by where it lies in relation to gas clouds and stars/galaxies post-collision. "Dark matter, which can be located via gravitational lensing, behaves somewhere on this continuum [between stars and gas]," the current study explains. "We have developed a statistical model to measure dark matter drag from many noisy observations, within which the relative trajectories of galaxies, gas, and dark matter can be combined in a way that eliminates dependence on 3D orientation and the time since the collision."

What that model found is that dark matter sailed on through about as much as the stars did. The dark matter core of one colliding galactic cluster basically ignored that of the other cluster. "It is unclear how much we expect dark matter to interact with itself because dark matter is already going against everything we know," offered David Harvey, the study's lead author, in a NASA statement. "We know from previous observations that it must interact with itself reasonably weakly, however this study has now placed it below that of two protons interacting with one another—which is one theory for dark matter."

So, the new study doesn't so much change what we know about dark matter as eliminate a set of theories about dark matter. We don't know know all that much about dark matter in the first place, just that it exists, we haven't been able to directly detect it, and that it comprises the vast majority of all matter in the universe. Elimination is progress.