Could the Milky Way’s most enigmatic light be the death cry of invisible matter? For over a decade, an explanation has eluded scientists for a diffuse gamma-ray glow emanating from the center of the galaxy, first detected in 2009 by NASA’s Fermi Gamma-ray Space Telescope. Called the Galactic Center GeV Excess (GCE), this high-energy radiation has two primary suspects: millisecond pulsars-the ultra-fast spinning remnants of massive stars-and dark matter particles annihilating each other.
Dark matter comprises about 27 percent of the universe’s mass-energy content and is visible only through its gravitational effects. One of the leading candidates for its composition is the Weakly Interacting Massive Particle, or WIMP. These hypothetical particles can pass through regular matter almost unaffected, but if a WIMP meets its antiparticle, they annihilate and release a cascade of particles, including gamma-ray photons. Such annihilation events would produce a smooth, diffuse glow across the sky, in contrast to the speckled texture expected from point sources like pulsars.
For years, morphology appeared to favor the pulsar explanation. The GCE is boxy in shape, which matches the distribution of old stars in the galactic bulge-the so-called X-shaped bulge. In contrast, the dark matter halos were presumed to be spherical, and such a boxy glow was thus inconsistent with annihilation models. However, that assumption has been overturned by new supercomputer simulations led by Moorits Mihkel Muru at the Leibniz Institute for Astrophysics Potsdam. The team reconstructed the Milky Way’s formation history, including billions of years of mergers with smaller galaxies, and found that the dark matter halo is not perfectly round but rather slightly flattened, or “egg-shaped.” That geometry naturally produces a boxy gamma-ray signal when viewed from our position about 8 kiloparsecs from the galactic center.
This refined model fits the Fermi telescope’s gamma-ray maps as well as pulsar-based predictions, and the researchers conclude from these observations that “both hypotheses for the GCE, that of dark matter annihilations and millisecond pulsars, are equally plausible based on morphology, spectrum, and intensity, with perhaps a slight edge for the dark matter hypothesis.” The edge comes from an observed deficiency in the number of millisecond pulsars needed to account for the glow’s intensity.
Gamma-ray astronomy has been critical to this debate. Instruments such as Fermi’s Large Area Telescope detect photons with energies in the GeV range, mapping their distribution across the sky. In the case of WIMP annihilation, theory predicts a sharp cutoff in the photon spectrum at an energy equal to the WIMP mass a spectral “edge” that would be a definitive signature of their presence. Detecting such a feature requires high sensitivity and resolution, capabilities that next-generation observatories will provide.
The CTAO now under construction in Chile and Spain will provide unprecedented gamma-ray imaging. By capturing Cherenkov light from particle showers triggered in Earth’s atmosphere, CTAO will resolve the GCE’s fine structure and energy spectrum far beyond Fermi’s capability. If the glow’s morphology is smooth and its spectrum matches the predictions for WIMP annihilation, the case for dark matter will strengthen dramatically. Conversely, a speckled pattern with higher energy emissions would point to pulsars.
Simulations with supercomputers have also expanded the search beyond the Milky Way. Predicting dark matter distributions among nearby dwarf galaxies allows researchers to compare these maps with gamma-ray observations, testing whether excesses like the one in the Milky Way occur. Such cross-checks could reveal whether the GCE is a unique feature of our galaxy or a universal signature of dark matter annihilation.
“Gamma rays, and specifically the excess light we’re observing at the center of our galaxy, could be our first clue.” says Joseph Silk, an astrophysicist at Johns Hopkins University. Yet he cautions that even with better data, “maybe we’ll find nothing, in which case it’ll be an even greater mystery to resolve.” The cosmic flip of the coin between pulsars and particles hasn’t been decided yet-but with the tools now coming online, astrophysicists are well-positioned to finally see which side lands face up.
