In this work we look at the complete cosmic microwave background (CMB) constraints on a millicharged dark matter (DM) model. Millicharged DM models have regained a lot of interest after the discovery of an anomalous 21cm signal by EDGES which can potentially be explained by the presence of scattering between DM and baryons. We focus on the simplest millicharged model that involves adding an unbroken U(1) symmetry to the Standard Model (SM). This addition to the SM adds a massive fermion that gets charged under the SM electric charge as well as a new gauge boson. The new gauge boson behaves as a massless dark photon.
Previous works mainly considered the implications from individual effects while we look at the impact of all of the interactions of the millicharged DM. We consider: scattering with photons, baryons and the dark photons as well as annihilations into SM particles. We rank the importance of each effect depending on region of the parameter space. By considering all of the effects we place constraints on the coupling of the DM to the standard model, the coupling of the dark matter to the dark photon as well as the redshift to the decoupling of the dark photons from the SM. We compare our cosmological constraints to these arising from other probes.