Could the universe’s most elusive substance finally have given away its presence? Nearly a century after the first indirect hints, a new analysis of data from NASA’s Fermi Gamma-ray Space Telescope offers a possible first direct detection of dark matter-through a faint but distinctive halo of high-energy light surrounding the center of the Milky Way.
The new study, led by Professor Tomonori Totani of the University of Tokyo, targets gamma rays at a photon energy of 20 gigaelectronvolts-20 billion electronvolts-an amount well beyond the realm of normal astrophysical processes. These emissions take on a spherically symmetric distribution, extending toward the galactic core in a halo-like shape. More importantly, the spatial morphology is consistent with theoretical models of the Milky Way’s dark matter halo-the vast, invisible structure believed to envelop our galaxy and dominate its mass.
Dark matter makes up approximately 85 percent of all matter in the universe but does not absorb, reflect, or radiate light and thus cannot be seen by standard optical telescopes. Its presence was first inferred in the 1930s by Swiss astronomer Fritz Zwicky, who observed that galaxies are moving at a speed too great for them to be held together by their visible mass. The favored candidate particles-weakly interacting massive particles, or WIMPs-are predicted to annihilate when colliding, sending out gamma-ray photons of specific energies. Totani’s detected spectrum rises from a few GeV, peaks sharply at 20 GeV, and falls again, matching the annihilation profile of WIMPs with masses about 500 times that of a proton.
The Fermi telescope, which launched in 2008, contains the Large Area Telescope, or LAT, a high-resolution gamma-ray detector that can scan huge swaths of the sky. By modeling and subtracting out known gamma-ray sources-including cosmic ray interactions, pulsars and the immense “Fermi Bubbles” of plasma above and below the galactic plane-Totani teased out a residual signal that couldn’t be explained by any known mechanism. “The shape of the gamma-ray emission component does closely match the shape expected from the dark matter halo,” he said, adding that no known cosmic-ray or stellar mechanism can create such a symmetrical energy spectrum.
The implications are huge: if confirmed, this will be the first-ever direct detection of dark matter and a particle beyond the Standard Model of particle physics. As Totani said, “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.”
Yet the excitement is tempered by caution. Experts such as David Kaplan of Johns Hopkins University emphasize that gamma rays can originate from numerous astrophysical sources, many still poorly understood. High-energy emissions might also come from fast-spinning neutron stars or black holes accreting matter, producing jets that mimic diffuse signals. Eric Charles of SLAC National Accelerator Laboratory emphasizes how hard it is to interpret gamma-ray data coming from big swaths of the sky-especially near the galactic center, which Boston University’s Dillon Brout calls “genuinely the hardest to model.”
Past claims of dark matter signatures, such as the galactic center GeV excess detected in 2009, have been left unresolved after years of study, with the improved models more often than not ascribing them to pulsars or other stellar sources. The origin of this less-studied halo-like 20 GeV excess is also not known. According to Jan Conrad of Stockholm University, “Because the backgrounds of astrophysical sources are so uncertain, it makes it very difficult to make strong claims.”
Independent verification is now essential. One of the most promising directions is to search for similar gamma-ray spectra in the dwarf galaxies orbiting the Milky Way. These nearby, small galaxies are rich in dark matter yet possess a very small astrophysical gamma-ray background, making them ideal testbeds. Detection of the same 20 GeV signal there would give strong support to the interpretation of dark matter. Another avenue lies with underground detectors designed to capture the rare WIMP interactions; a matching particle mass from such experiments would provide a strong cross-check. Future instrumentation could sharpen the picture.
The forthcoming Cherenkov Telescope Array Observatory will increase sensitivity to high-energy gamma rays by an order of magnitude, allowing far more detailed mapping of the halo excess. Until then, Totani’s result is a tantalizing possibility-one that could revolutionize cosmology if the faint glow in the halo of the Milky Way really does emanate from the annihilation of dark matter particles.
