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Dark Matter Gets a Little Darker

A unique and powerful gamma-ray observatory scans nearby galaxies for telltale signatures of dark-matter particles, but dark matter proves elusive yet again.
March 1, 2018
A galaxy in space; other galaxies dot the background.

UGC 12591 is the fastest spinning spiral galaxy known, with stars orbiting the center about twice as fast as in the Milky Way. Stellar orbital speeds-in this galaxy and pretty much every other-are too fast; without additional gravity from a great deal of unseen dark matter, galaxies shouldn't be able to keep their stars from flying away. CREDIT: NASA, ESA, Hubble

"If we have to, we'll find out what dark matter is by crossing off every single thing that it isn't." - Andrea Albert, Los Alamos astrophysicist

Perched on the slope of a volcano in Mexico, the third-highest peak in North America, the High-Altitude Water Cherenkov (HAWC) Gamma-ray Observatory 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 HAWC’s water tanks. It just so happens that—in theory, at least—dark matter particles that collide with one another or decay should produce very-high-energy gamma rays capable of triggering this event.