Hunting Dark Matter Lines in the Infrared Background with the James Webb Space Telescope

Dark matter particles with a mass around 1 eV can decay into near-infrared photons. Utilising available public blank sky observations from the NIRSpec IFU on the James Webb Space Telescope (JWST), we search for a narrow emission line due to decaying dark matter and derive leading constraints in the mass range 0.8-3 eV on the decay rate to photons, and more specifically, on the axion-photon coupling for the case of axion-like particles. We exclude $\tau <6.7\cdot {10}^{26}$ s at $m_{\mathrm{D}\mathrm{M}}\simeq 0.9$ eV and, in the case of axions, $g_{a\gamma \gamma }>9.4\cdot {10}^{-12}$ GeV${}^{-1}$ for $m_a=2.15$ eV. Our results do not rely on dedicated observations, rather we use blank sky observations intended for sky subtraction, and thus our reach may be automatically strengthened as JWST continues to observe.