In the early 1930s, Swiss astronomer Fritz Zwicky observed galaxies in space moving faster than their mass should allow, prompting him to infer the presence of some invisible scaffolding — dark matter — holding the galaxies together. Nearly 100 years later, NASA's Fermi Gamma-ray Space Telescope may have provided direct evidence of dark matter, allowing the invisible matter to be "seen" for the very first time.
Dark matter has remained largely a mystery since it was proposed so many years ago. Up to this point, scientists have only been able to indirectly observe dark matter through its effects on observable matter, such as its ability to generate enough gravitational force to hold galaxies together. The reason dark matter can't be observed directly is because the particles that make up dark matter don't interact with electromagnetic force — meaning dark matter doesn't absorb, reflect or emit light.
Theories abound, but many researchers hypothesize that dark matter is made up of something called weakly interacting massive particles, or WIMPs, which are heavier than protons but interact very little with other matter. Despite this lack of interaction, when two WIMPs collide, it is predicted that the two particles will annihilate one another and release other particles, including gamma ray photons.
Researchers have targeted regions where dark matter is concentrated, such as the center of the Milky Way, through astronomical observations for years in search of these specific gamma rays. Using the latest data from the Fermi Gamma-ray Space Telescope, Professor Tomonori Totani from the Department of Astronomy at the University of Tokyo believes he has finally detected the specific gamma rays predicted by the annihilation of theoretical dark matter particles.
Totani's study is published in the Journal of Cosmology and Astroparticle Physics.
"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 observed energy spectrum, or range of gamma-ray emission intensities, matches the emission predicted from the annihilation of hypothetical WIMPs, with a mass approximately 500 times that of a proton. The frequency of WIMP annihilation estimated from the measured gamma-ray intensity also falls within the range of theoretical predictions.
Importantly, these gamma-ray measurements are not easily explained by other, more common astronomical phenomena or gamma-ray emissions. Therefore, Totani considers this data a strong indication of gamma-ray emission from dark matter, which has been sought for many years.
"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.
While Totani is confident that his gamma-ray measurements are detecting dark matter particles, his results must be verified through independent analysis by other researchers. Even with this confirmation, scientists will want additional proof that the halolike radiation is indeed the result of dark matter annihilation rather than originating from some other astronomical phenomena.
Additional proof of WIMP collisions in other locations that harbor a high concentration of dark matter would bolster these initial results. Detecting the same energy gamma-ray emissions from dwarf galaxies within the Milky Way halo, for example, would support Totani's analysis.
"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.
