Scientists may have found dark matter after 100 years of searching

Dark matter has remained one of astronomy's biggest unknowns since it was first suggested. Until now, scientists have only been able to study it indirectly by observing how it affects ordinary matter, such as the way it produces enough gravity to hold galaxies together. Direct detection has not been possible because dark matter particles do not interact with electromagnetic force -- meaning they do not absorb, reflect or emit light.

The WIMP Hypothesis and Predicted Gamma Rays

Many researchers believe that dark matter is made of weakly interacting massive particles, or WIMPs. These particles are thought to be heavier than protons and interact so weakly with normal matter that they are extremely difficult to detect. However, theory suggests that when two WIMPs collide, they annihilate each other and release energetic particles, including gamma ray photons.

Scientists have spent years examining regions where dark matter should be concentrated, especially the center of the Milky Way, searching for these specific gamma rays. Using new data from the Fermi Gamma-ray Space Telescope, Professor Tomonori Totani of the University of Tokyo now believes he has identified the predicted gamma ray signal associated with dark matter particle annihilation.

Totani's findings appear in the Journal of Cosmology and Astroparticle Physics.

A 20-GeV Gamma Ray Halo Near the Milky Way Center

"We detected gamma rays with a photon energy of 20 gigaelectronvolts (or 20 billion electronvolts, an extremely large amount of energy) extending in a halolike structure toward the center of the Milky Way galaxy. The gamma-ray emission component closely matches the shape expected from the dark matter halo," said Totani.

The measured gamma ray energy spectrum, which describes how the intensity of the emission varies, closely matches model predictions for the annihilation of hypothetical WIMPs with masses roughly 500 times that of a proton. The estimated frequency of these annihilation events based on the observed gamma ray intensity also fits within expected theoretical ranges.

Evaluating the Possibility of a Major Breakthrough

Totani explains that the gamma ray pattern cannot be easily matched to other known sources or more common astrophysical processes. Because of this, he views the data as a strong candidate for long-sought gamma ray emission from dark matter.

"If this is correct, to the extent of my knowledge, it would mark the first time humanity has 'seen' dark matter. And it turns out that dark matter is a new particle not included in the current standard model of particle physics. This signifies a major development in astronomy and physics," said Totani.

Next Steps and Independent Verification

Although Totani is confident in his analysis, he emphasizes that independent confirmation is essential. Other researchers will need to review the data to verify that the halolike radiation truly results from dark matter annihilation rather than another astrophysical source.

Further support could come from finding the same gamma ray signature in other regions rich in dark matter. Dwarf galaxies orbiting within the Milky Way halo are considered especially promising. "This may be achieved once more data is accumulated, and if so, it would provide even stronger evidence that the gamma rays originate from dark matter," said Totani.

Funding: This work was supported by JSPS/MEXT KAKENHI Grant Number 18K03692.