A weird, unidentified X-ray signal radiating out of nearby galaxies has perplexed scientists ever since it was first detected in 2014. Now, a promising study into its origins has come up empty-handed, meaning that whatever is causing the faint signature is currently unknown to us and more mysterious than ever.

The origins of the faint glow, which emits 3.5 kiloelectronvolts (keV) of energy, are especially intriguing because the signal matches key predictions about the nature of dark matter, a mysterious substance from which most of the universe is made. For years, scientists have debated whether the glow was long-sought evidence of a hypothetical particle that makes up dark matter. A clear answer to that question could finally unravel the true nature of dark matter, which is the “holy grail of astroparticle physics at the moment,” said Benjamin Safdi, an astrophysicist at the University of Michigan, in a call.

Well, Safdi and his colleagues have good and bad news on that front. The good news is that the researchers pioneered an innovative new method to search for dark matter around our home galaxy, the Milky Way, which is outlined in a study published on Thursday in Science.

The bad news is that the hunt came up empty, and that effectively rules out dark matter as the source of the signal. “On the one hand, this was not the outcome that we were hoping for,” Safdi said. “We were certainly hoping to discover dark matter.”

“On the other hand, we’re really excited that we now have this better method to look for dark matter,” he added. “We didn’t discover dark matter this time, but there's no reason to think that we won’t in the future.”

Scientists know that dark matter exists because they can see its gravitational pull on galaxies and other radiant objects. But because this form of matter does not emit light, researchers are literally left in the dark about most of its properties.

Many theoretical models have been proposed to explain dark matter, such as the existence of a particle called a sterile neutrino. Scientists have suggested that sterile neutrinos might slowly decay in a somewhat similar process to radioactive particles here on Earth.

“We know that dark matter is pretty stable because it was created at the Big Bang and it’s still around today billions of years later," Safdi explained. “If it decays, it must decay very slowly.”

Representation of X-ray emission line concentrated around the center of the Milky Way (in black and white). Image: Zosia Rostomian and Nicholas Rodd/Berkeley Lab; and Christopher Dessert and Benjamin Safdi/University of Michigan

Even with a long half-life, this speculative decay of sterile neutrinos might produce a very small amount of light, which would mean that dark matter is not completely black. The 2014 study, which was published in The Astrophysical Journal, appeared to detect exactly this type of signature, causing a major splash in the dark matter research community.

“There was pretty intense debate, over the years following this paper, trying to understand whether or not this emission was coming from dark matter or ordinary matter within these galaxies,” Safdi said.

Safdi had assumed that the question would be cleared up by the slick observational capabilities of Japan’s Hitomi satellite, an X-ray observatory that launched in 2016. But due to multiple malfunctions, Hitomi fell apart after just six weeks in space, and no equivalent replacement for it has filled the gap in X-ray observations since.

Though the loss was disappointing, Safdi and his colleagues soon began ruminating on other ways to study the 3.5 keV emission line. “We asked the question: What can we do with the existing data?” he recalled. “We realized that there was a very natural analysis to do with the data from the XMM Newton space telescope, which has been in the sky for over 20 years.”

The team searched through every image and datasat captured by XMM Newton, launched in 1999 by the European Space Agency. By removing the portions of images that contained luminous objects, Safdi and his colleagues were able to analyze two decades of negative “blank” sky.

If sterile neutrinos were decaying around the Milky Way, the signature would have been extremely bright in this dataset. Alas, the search revealed only empty space.

“That tells us very definitively that this line that was observed in other galaxies is not coming from dark matter decay,” Safdi said. “But that doesn’t mean that the 3.5 keV line doesn’t exist. Likely what is causing that emission is some currently unknown process going on within those galaxies that has to do with the ordinary matter, not the dark matter.”

It’s possible that the 3.5 keV emission is caused by specific elements within the hot gases of the galaxy clusters, or interactions between hot plasmas and cold gas clouds, according to the study.

Ultimately, the new results leave scientists with two mysteries to solve: the origins of this eerie glow and the true identity of dark matter. Safdi and his colleagues are optimistic that novel techniques and next-generation observatories will be able to constrain both of these tantalizing questions.