Controversial New Study Points to the Most Promising Dark Matter Signal Yet

Astronomers have spent nearly a century searching for dark matter, the invisible scaffold thought to hold galaxies together. Although there is abundant indirect evidence to suggest the existence of this mysterious substance, no one has been able to detect it directly. Now, a new study may finally indicate a breakthrough.

Using data from NASA’s Fermi Gamma-ray Space Telescope, astronomer and University of Tokyo professor Tomonori Totani claims to have identified gamma-ray emissions that appear to originate from dark matter. Their findings, published Tuesday in the Journal of Cosmology and Astroparticle Physics, show that this radiation was emitted by colliding WIMPs (weakly interacting massive particles).

“A leading candidate for dark matter, WIMPs have long been predicted to annihilate and emit gamma rays, prompting numerous search efforts,” Totani told Gizmodo in an email. “This time, using the latest Fermi satellite data accumulated over 15 years and a new method focusing on the halo region (except the galactic center), I have discovered gamma-ray emission that originates from dark matter.”

It’s an interesting finding, but the experts we spoke to were unconvinced, warning that the signal could be a case of cosmic noise mistaken for dark matter or another frustrating false positive.

Totani himself emphasizes that it is too early to say with certainty that these gamma rays originated from dark matter, but their characteristics suggest that they could. Based on their findings, they do not look like those that have arisen from traditional astronomical sources. “At the very least, it represents the most promising candidate radiation from dark matter known so far,” he said.

Finding a Needle in a Cosmological Haystack

Astronomers believe dark matter exists because no other observable substance in the known universe can explain certain gravitational effects, such as galaxies rotating unexpectedly faster or the fact that they are bound together more tightly than they should be.

Dark matter is the theoretical answer to this cosmological puzzle, but if it exists, its particles apparently do not absorb, reflect, or emit light. If they had done so, astronomers would have discovered this abundant substance long ago.

WIMPs largely fit that description. Astronomers believe that WIMPs interact through gravity, but their interactions with electromagnetic and nuclear forces are too weak to detect. However, when they collide with each other, they should theoretically annihilate and emit gamma rays.

Researchers have searched for these gamma-ray emissions for years, and have targeted regions of the galaxy where dark matter appears to be concentrated, such as the galactic center. These searches came up empty, so Totani decided to look elsewhere, specifically in the galaxy’s halo region.

This extended, roughly spherical region surrounding the Milky Way’s galactic disk contains stars, gas, and possibly large amounts of dark matter. By analyzing Fermi satellite observations of the halo, Totani identified high-energy gamma ray emissions that align with the shape expected from a dark matter halo.

The range of gamma-ray emission intensity they observed matches what astronomers expect to see from a WIMP annihilation. Totani also estimated the frequency of WIMP destruction from the measured gamma-ray intensity, and this also falls within the range of theoretical predictions. This increases the possibility that it may have detected a signal generated by dark matter WIMPs.

Case closed? not yet

The findings are encouraging, but Totani and other experts caution that these gamma rays are not a smoking gun.

“The problem is that there are so many ways to make gamma rays, everything from pulsars to black holes to supernovae to inspiring matter,” a Fermilab physicist told Gizmodo. “Hey, we get gamma rays from the Sun.”

Fermilab officials asked Gizmodo to refrain from naming the scientist who provided the quotes.

What sets the gamma rays discovered by Totani apart from most others is how energetic they are, with a photon energy of 20 gigaelectronvolts. That’s “pretty massive,” the Fermilab physicist explained, but not entirely unheard of. “There are a lot of highly energetic things in space, and those highly energetic things can create high-energy gamma rays.”

While the gamma emissions found by Totani fit the description of emissions resulting from the destruction of a WIMP, there are other possible explanations that should be ruled out first, according to the Fermilab physicist. These could include high-energy events such as neutron star collisions or solar winds from pulsars, he said.

Additional studies will also be needed to validate Totani’s observations and calculations. “Conclusive proof will be the detection of gamma rays with similar dark matter parameters from other areas of the sky,” Totani said. “I hope that these results will be verified by independent analyzes conducted by other researchers.”

With that being said, Dan Hooper, professor of physics at the University of Wisconsin–Madison and director of the Wisconsin IceCube Particle Astrophysics Center, points out that several other scientists have already analyzed the Fermi satellite data Totani used, and none have detected additional gamma ray emissions.

“Now, there are a few different options out there, and I’m glad people are trying different things, but it doesn’t give me a lot of confidence that this is an authentic sign of dark matter,” Hooper told Gizmodo.

For one thing, Totani didn’t look for gamma rays anywhere within 10 degrees of the galactic center. While this approach may offer some advantages, avoiding the galactic center could impact the findings, because this region of our galaxy is where physicists expect a large portion of the dark matter signal to come from, Hooper explained.

They also suspect that the high-energy gamma ray emission detected by Totani may actually be an artifact of the analysis. This may result from using a background model that is absorbing too much emission at low energies, creating the illusion of higher-energy excesses.

The bottom line is that “dark matter is very hard to find, it is very hard to characterize it,” said the Fermilab physicist. “Nobody should believe this without multiple mutually valid lines of evidence, and this is just one.”

So, the search for dark matter continues. Whether future studies confirm or undermine Totani’s findings remains to be seen, but either way, they will help researchers refine our understanding of the invisible matter that shapes our universe.