Recent measurements of diffuse sub-PeV $\gamma$ rays by the Tibet ${\mathrm{AS}}_{\gamma }$ and Large High Altitude Air Shower Observatory (LHAASO) collaborations have reshaped our understanding of the $\gamma$-ray sky. Besides uncovering the nature of “PeVatrons,” these measurements can also be used to probe the nongravitational nature of dark matter. PeV-scale decaying dark matter can produce high-energy $\gamma$ rays in the final state and contribute to the measurements made by extensive air-shower detectors such as Tibet ${\mathrm{AS}}_{\gamma }$ and LHAASO. Using the latest Tibet ${\mathrm{AS}}_{\gamma }$ upper limits on diffuse $\gamma$ rays away from the Galactic plane and the LHAASO square kilometer array (LHAASO-KM2A) measurements of diffuse $\gamma$ rays from the Galactic plane, we put stringent constraints on lifetimes of decaying dark matter for masses $\sim {10}^6–{10}^9\mathrm{GeV}$. Future observations of high-energy diffuse $\gamma$-ray emission can thus provide stronger limits or potentially discover heavy decaying dark matter.

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• Dark matter
• Gamma ray astronomy
• Particle dark matter