Cosmological Limits on Strong Yukawa Dark Forces

We showcase cosmology's ability to constrain long-range forces between dark matter particles. Specifically, we consider a fermionic dark matter interacting via a Yukawa-coupled light scalar, focusing on regimes where the dark forces are stronger than gravitational and yet unconstrained. We show that the dark-sector dynamics---both at the background and perturbation levels---is far richer than what can be captured with static interparticle Yukawa potential. The background dynamics includes an attractor that funnels a wide range of initial conditions onto an evolution unique to each parameter space. In a large swath of parameter space beyond existing limits, the dark sector deviates drastically from cold dark matter. We rule out this parameter space using existing constraints on dark-matter equation of state and small-scale cosmic perturbations.