Comparison of ULA constraints obtained from eRASS1 galaxy cluster abundance (this work), Planck 2015 (Hložek et al. 2018), and 2018 CMB data; BOSS galaxy clustering data (both by Rogers et al. 2023); and combined analyses of eRASS1 and Planck 2015 (this work) and Planck 2018 and BOSS (Rogers et al. 2023). The bar height indicates the 95% exclusion limits of the ULA density Ω_a times h² = (H₀/100)² in each log₁₀(m_a[eV]) bin. eRASS1 yields the tightest upper bounds on dark matter ultralight axions in the log₁₀(m_a[eV]) ∈ { − 27, −26} bins.