The Universe, according to cosmologists, is divided into three unequal portions. In the big bargain bin near the exit aisle, you’ll find dark energy. Dark energy’s job is to push everything apart. There’s lots of it, and no one knows why. Then, next to dark energy, on a display shelf, you’ll find a smaller amount of dark matter. Regular matter, the stuff you and I can see, is tucked off to the side: small and barely noticeable.

But the Universe isn't a store's display, and these individual parts can interact. Many of our models for dark matter predict a hierarchy of structures—halos of dark matter in and around galactic clusters and individual galaxies that hold them together. At large scales, we can detect these halos through gravitational lensing. Because there are observations to constrain them, all current theories of dark matter predict large halos correctly.

However, these same theories also predict smaller halos with sizes between 1,000 and a billion solar masses. In this mass range, we have no observations, and consequently, current theories disagree. These tiny halos are going to be hard to spot, though: their gravitational effect—the only way we can see them—will be minimal. But, now a group of researchers may have figured out a new way to spot some of them.

Hunting for the dark

To spot dark matter, we have to turn to the third and smallest portion of the Universe: normal matter. Yes, by cosmological error margins, everything that can be touched, felt, or seen is pretty much negligible. Cosmologists don’t neglect the negligible, though, because that tiny fraction of the Universe is all we can detect. We only know about dark energy and dark matter through how they change the motion of visible matter.

This change in motion is exactly what researchers predict will happen when small dark matter halos pass close to clumps of ordinary matter. A cloud of dark matter passing through a galaxy turns out to be a bit like a spoon stirring coffee. The resulting disruption to the galaxy should be detectable as a kind of wake through the stars, according to researchers.

Picture the inner part of a galaxy: millions of stars that have been orbiting around each other for billions of years. In that time they have kicked and pulled at one another so that they have reached a kind of equilibrium. As a result, the distribution of velocities follows a specific mathematical form. We can even measure that distribution by looking at the doppler shift of the color of light from different stars in the galaxy.

From stage left, like an unimaginably huge spoon, a dark matter halo passes through the inner galaxy, stirring the stars. Those stars that were moving in the same direction as the halo get accelerated to even higher speed; those moving in the opposite direction get slowed. Stars outside the immediate vicinity of the halo get drawn toward it. In other words, the distribution of stellar velocities gets seriously distorted.

Crossing the streams

The researchers' calculations show that it should be possible to measure the stellar wake from dark matter halos down to about 10 million solar masses. This happens to fit beautifully in between two other measuring techniques: gravitational lensing can pick up the bigger halos, and one called the stellar stream method may be sensitive to even smaller halos than the stellar wake technique.

The stellar wake’s stirring motion provides a different view to that of the stellar stream, though. As the halo passes through the galaxy, the stars’ motions change, and that can be measured as soon as the light reaches the Earth. The stellar stream method relies on the halo first disturbing a stream of stars, which then wiggles about, producing gaps in the stream. It takes time for these gaps to form.

That means we don’t get to see the dark matter disturbing the galaxy now—it's obvious only in the effect after the halo has long gone. If we want to observe the process of dark matter mixing up a galaxy, the stellar wake method may be the way to go.

Normally, I don’t write about proposed measurements too often because of the time lags involved. The first paper starts an argument. Assuming that goes well, someone has to propose a space mission. This has to go through umpteen billion rounds of review before the completed satellite is blown up on the launch pad. Two decades later, the rebuilt satellite finally starts taking data.

This latest proposal avoids that. The stuff we want to analyze can be obtained using a combination of preexisting data from the likes of the Sloan digital sky survey and from surveys that are currently underway (like the Gaia mission) or are about to take place. No new instruments or experiments required, just lots and lots of data analysis.

As with most experiments, this will not eliminate all competing dark matter theories, leaving us with the one true explanation. It will, however, narrow the field considerably and give us a better understanding of how the structures of the Universe fit together with each other. And, if it turns out that these tiny halos don’t exist—or are undetectable at any rate—that, too, will tell us something.

Physical Review Letters, 2018, DOI: 10.1103/PhysRevLett.120.211101. (About DOIs).