Dark Matter Gets a Little Darker

In the 1970s, astronomers realized that galaxies are filled with dark matter: matter that doesn't emit, absorb, or block light (like stars, gas, or dust) but still exerts a gravitational force, as measured by the orbital speeds of stars and other objects circling around the centers of galaxies. For a typical galaxy like our own Milky Way, there is about ten times more dark matter than all the regular forms of matter combined.

Four decades of intensive scrutiny later, the dark matter, undeniably present, still hasn't been identified. The leading contender for what might comprise it, or at least most of it, is a yet-to-be-identified elementary particle that swarms invisibly in tremendous numbers throughout galaxies and other astronomical structures. These particles could be passing unhindered through our planet right now, gliding easily through the empty spaces inside atoms, despite having enough mass to generate a powerful collective gravitational pull.

Such a weakly interacting massive particle (WIMP) could account for the majority of dark matter (with comparatively little dark matter in the form of chunkier things like black holes and very dim stars). But if a WIMP can pass clean through the earth, then it can also pass clean through the specialized machines that earthlings build to detect it. Nonetheless, a number of dark matter detectors have been built, running for years on end and accumulating enough non-detections to rule out various categories of WIMPs and other dark matter candidates. Key among the categories that remain are particles somewhat heavier than conventional WIMP candidates; these could be detectable by an unconventional detector.

Astrophysicist Andrea Albert analyzes data from the High-Altitude Water Cherenkov Gamma-ray Observatory (HAWC), a novel Los Alamos-led gamma-ray and cosmic-ray telescope (see Celestial Mystery Machine the May 2015 issue of 1663). Perched on the slope of a volcano in Mexico, the third-highest peak in North America, HAWC is not a system of lenses and mirrors but a collection of ultrapure water tanks wired up with ultrasensitive light detectors. Whenever a very-high-energy particle from space collides with an air molecule overhead, a spray of subatomic particles rains downward and generates a faint flash of blue light in HAWCs water tanks. It just so happens that in theory, at least WIMPs that collide with one another or decay should produce very-high-energy gamma rays capable of triggering this event.

"HAWC was not designed solely to look for dark matter," Albert says, "but it is extremely well suited for picking up gamma-ray signals from higher-mass dark matter particles." Unlike conventional astronomical observatories, HAWC is almost always on, day and night. And it doesn't need to be pointed at any single astronomical object, but rather takes in many targets at once as it consistently sweeps across two-thirds of the sky over the course of the earths daily rotation. In addition, it is the only instrument in operation that probes for dark matter particles beyond a particular mass threshold, about 20,000 times the mass of a proton.

So, has HAWC spotted any dark matter? Well, no, it did what every other detector has done for decades: it ruled out some types of dark matter based on what it didn't see. But strangely, HAWCs strongest constraints on dark matter come from gamma rays it didn't see from a galaxy that didn't exist, as far as anyone knew.

"We looked for gamma-ray dark-matter signals from a number of astronomical sources, including our neighbor the Andromeda Galaxy," Albert says. "The most constraining results came from observations of dwarf galaxies, and our strongest result came from Triangulum II, a dwarf galaxy so faint that it hadn't even been discovered until after we pooled our dwarf-galaxy data. Luckily, because of HAWCs wide field of view and nearly 100 percent up time, we already had data on it."

Dwarf galaxies are what they sound like, typically with only a few hundreds of millions of stars, as contrasted with a full-fledged galaxy like the Milky Way, with stars numbering in the hundreds of billions. Triangulum II appears to contain only about a thousand. They are gravitationally bound together by an unprecedented excess of dark matter, with thousands of times more of it than normal matter. This is a double virtue for Triangulum II and dwarf galaxies in general: more dark matter to collide or decay, producing gamma rays, and fewer normal-matter sources, such as supernovae, to produce gamma rays of their own.

In all, Albert and colleagues examined 15 nearby dwarf galaxies and found no gamma rays that could be attributed to dark matter. But as any dark-matter hunter knows by now, the search doesn't stop there. Over time, more gamma-ray data will be collected from these regions, and, with enough patience, a faint signal may eventually emerge. For now, however, there is only the slow march of determined science, a rigorous and gradual chipping-away process, producing ever-tighter limits on the properties that dark matter could possibly have. In the upper-mass range, it can only have x propensity to collide or y rate of decay.

"Sometimes in science you have to accept a process of elimination," Albert says. "If we have to, well find out what dark matter is by crossing off every single thing that it isn't."