A leading theory, NASA reports, is that the X-rays are a result of decaying "sterile neutrinos," which are particles that could potentially produce dark matter. Astronomers believe dark matter accounts for 85 percent of all matter in the universe, but dark matter does not emit or absorb light as does the "normal" matter in planets, stars, and galaxies, so its detection is much more difficult.

NASA notes that normal matter may also have created the emission line, which appeared in the Perseus galaxy cluster, but astronomers are excited at the prospect that the sterile neutrinos may be at play. The scientists' next step will be combining the data with results from other missions to see if the same X-ray signal appears in other galaxy clusters. Thus far, the line has also been found in 73 other galaxy clusters using NASA's XMM-Newton.

"We know that the dark matter explanation is a long shot, but the pay-off would be huge if we're right," Esra Bulbul, leader of the study and a fellow at the Harvard-Smithsonian Center for Astrophysics, told NASA.