Turning the cosmic web into a detector: first limits set on dark matter decaying into gravitons

Physicists have found a way to use the vast filaments that thread the universe together as a giant detector for one of the strangest ideas in fundamental physics: that dark matter might slowly decay into gravitons, the hypothetical carriers of gravity itself. A team led by David Dunsky at New York University has set the first observational limits on how fast such a process could occur, using the faint gamma-ray glow of the sky. The work was published in Physical Review D.

Two invisibles at once

Dark matter makes up roughly 85% of the universe’s mass, yet it reveals itself only through gravity. Its true nature has escaped physicists for decades, and one open possibility is that it is slightly unstable — that its particles gradually decay over cosmic timescales. Dunsky’s team examined a particularly exotic version of that idea: dark matter decaying into gravitons.

That would seem to trade one undetectable thing for another. Gravitons, like dark matter, have never been observed, and catching a single one directly may be effectively impossible because gravity’s pull on individual particles is so extraordinarily weak. But there is a loophole. Through a known process called the Gertsenshtein effect, a graviton passing through a magnetic field has a tiny chance of converting into a photon — a particle of light we can actually detect.

Why the filaments are the ideal stage

The trick is finding somewhere with magnetic fields vast enough to give that rare conversion a meaningful chance to happen. Dunsky’s team realized the best stage is the cosmic web itself: the network of filaments connecting galaxies across the observable universe. These filaments carry weak but remarkably far-reaching magnetic fields that stay coherent across millions of light-years, and collectively they fill a large fraction of the cosmos. If dark matter were decaying into gravitons anywhere along them, the resulting graviton-to-photon conversions would seed a faint, steady glow of gamma rays spread across the sky.

Crucially, this glow would come mostly from distant intergalactic filaments — not from the crowded center of our own galaxy, where most dark matter searches traditionally point. And the method leans on no exotic new physics beyond the graviton-decay hypothesis itself; the light-conversion step rests on established theory.

A non-detection that still constrains

To test the idea, the team compared their prediction against the diffuse gamma-ray background measured across the whole sky by NASA’s Fermi-LAT space telescope. No unexplained excess appears in that data — and that absence is itself informative. From the non-detection, the researchers derived the first limits on how quickly dark matter could be decaying into gravitons, across an enormous range of possible dark matter masses.

The strategy is only going to get sharper. Dunsky’s team notes that a proposed successor to Fermi, the Advanced Particle-astrophysics Telescope, could tighten these constraints by a factor of ten. If dark matter really does decay into gravitons, scanning the magnetized filaments of the cosmic web may turn out to be the most sensitive way yet to catch the faint light it leaves behind.

© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved. Publication: David I. Dunsky et al., Observing dark matter decays to gravitons via graviton-photon conversion, Physical Review D (2026). DOI: 10.1103/yvs5-67cj