A mysterious glow detected in the outer regions of the Milky Way may provide the first clues about the nature of dark matter, yet astronomers caution that it’s premature to draw any definitive conclusions.

Dark matter is theorized to account for 85% of the universe’s total mass, but scientists have struggled to identify the particles constituting it.

Among the potential candidates for dark matter are weakly interacting massive particles (WIMPs). These elusive particles are notoriously hard to detect as they seldom interact with normal matter but are believed to occasionally self-annihilate, creating bursts of high-energy radiation in the form of gamma rays.

If dark matter is uniformly distributed across the galaxy as indicated by its gravitational effects, and if it consists of WIMPs, we should observe gamma rays as these particles self-annihilate. For over a decade, astronomers have been investigating whether the anomalously high gamma-ray emissions from the galactic center could signal this phenomenon, yet conclusive evidence remains elusive.

Now, Tomonori Toya, a professor at the University of Tokyo, claims he may have detected such a signal emanating from the Milky Way’s outer halo, utilizing 15 years’ worth of observations from NASA’s Fermi Gamma-ray Space Telescope.

Toya devised a model predicting the expected gamma-ray radiation in this region based on established sources like stars, cosmic rays, and vast bubbles of radiation identified above and below the Milky Way. Upon subtracting this known radiation from the total observed by Fermi, he found a residual gamma-ray glow with an energy level around 20 gigaelectronvolts.

This specific gamma-ray energy strongly aligns with the theoretically anticipated emissions from WIMPs’ self-annihilation, according to Toya. Although he admits it is too early to assert that these gamma-ray spikes are definitively due to dark matter, he describes the findings as “the most promising candidate for radiation from dark matter known to date.”

“Though the research began with the aim of identifying dark matter signals, I initially felt skeptical—like winning the lottery. When I first observed what seemed to be a signal, I approached it with caution,” says Totoni. “However, after thoroughly checking everything and confirming its accuracy, I was filled with excitement.”

“This represents a significant result worthy of further investigation, but firm conclusions cannot be drawn at this stage,” states Francesca Karoly from the French National Center for Scientific Research in Annecy. Accurately modeling all gamma-ray sources in the Milky Way, aside from dark matter, is quite complex, and Totoni has yet to deeply validate her models.

Silvia Manconi of France’s Sorbonne University asserts that the results need additional scrutiny, and more robust models are essential to establish whether the signals are genuine. Additionally, gamma-ray signals from other sources, like dwarf galaxies, are still unobserved and require thorough explanation, she mentions.

Many alternative radiation sources, including radio waves and neutrinos, will also need analysis to ensure the gamma rays aren’t being attributed to something else, says Anthony Brown from Durham University, UK. “Analyzing from just one perspective isn’t sufficient,” he states. “Dark matter necessitates an abundance of high-quality data.”

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