Limit on the dark matter mass from its interaction with photons

In this work, we explore the phenomenology of generalized dark matter (GDM) which interacts with photons (). We assume that DM establishes elastic scattering with when it has already become nonrelativistic, otherwise the abundance of DM today is disfavored by current observations. Within this scenario, the equation of state (EoS) of DM is determined by its mass (m_) and the DM- scattering cross section. The distinctive imprints of a nonzero EoS of DM on CMB angular power spectrum allow us to set a lower limit on m_ with Planck 2018 data alone, i. e. , m_>8. 7 keV at 95% C. L. In the study of cosmic concordance problems, we find that the GDM scenario preserves the sound horizon (rₒ (z*) ) predicted in the fiducial model, and thus does not solve the H₀ tension. When performing the joint analysis of Planck+LSS datasets, the best-fit S₈=0. 7850. 017 closely matches the given S₈ prior. This suggests that the GDM scenario can be counted as a viable candidate to restore the S₈ (₈) tension.