We stack 3.75 Ms of early XRISM Resolve observations of 10 galaxy clusters to search for unidentified spectral lines in the E = 2.5─15 keV band (rest frame), including the E = 3.5 keV line reported in earlier low spectral resolution studies of cluster samples. Such an emission line may originate from the decay of the sterile neutrino, a warm dark matter (DM) candidate. No unidentified lines are detected in our stacked cluster spectrum, with the 3σ upper limit on the ms ∼ 7.1 keV DM particle decay rate (which corresponds to an E = 3.55 keV emission line) of Γ ∼ 1.0 × 10−27 s−1. This upper limit is 3─4 times lower than the one derived by Hitomi Collaboration from the Perseus observation but still 5 times higher than the XMM-Newton detection reported by E. Bulbul et al. in the stacked cluster sample. XRISM Resolve, with its high spectral resolution but small field of view, may reach the sensitivity needed to test the XMM-Newton cluster sample detection by combining several years worth of future cluster observations.
