Ruling out strongly interacting Dark Matter–Dark Radiation models from joint observations of Cosmic Microwave Background and Quasar absorption spectra

The cold dark matter (CDM) paradigm provides a remarkably good description of the Universe’s large-scale structure. However, some discrepancies exist between its predictions and observations at very small sub-galactic scales. To address these issues, the consideration of a strong interaction between dark matter particles and dark radiation emerges as an intriguing alternative. In this study, we explore the constraints on those models using joint observations of Cosmic Microwave Background (CMB) and Quasars absorption spectra with our previously built parameter estimation package CosmoReionMC. At 2-σ confidence limits, this analysis rules out the strongly interacting Dark Matter - Dark Radiation models within the recently proposed ETHOS framework, representing the most stringent constraint on those models to the best of our knowledge. Future research using a 21-cm experiment holds the potential to reveal stronger constraints or uncover hidden interactions within the dark sector.