Exclusion regions (95% confidence level) in the log₁₀(m_a[eV]) − Ω_ah² space as constrained by different probes. Colored regions represent the part of the parameter space where one ULA species can be excluded at 95% confidence level, whereas white regions represent the unconstrained parameter space. eRASS1 galaxy cluster number counts (orange) exclude a new region in the parameter space between 10^−27.5 eV ≲ m_a ≲ 10⁻²⁶ eV, corresponding to galaxy cluster scales. Combining eRASS1 galaxy cluster number counts with the Planck 2015 CMB data (red) further improves the eRASS1 bounds on the ULA relic density. For ULA masses below m_a ≲ 10⁻²⁸ eV, Planck 2018 CMB data yields the tightest bounds. Above m_a ≳ 10⁻²⁵ eV, galaxy clustering data from BOSS yields the tightest constraints. The curves are interpolated between the central bin values, which represent the 95% confidence levels in the respective bins. In the m_a = 10⁻²⁵ eV mass bin, the Planck upper confidence level has been assumed as a reasonable choice for the interpolation (for details on that bin, see Sect. 5.2).