Self-interacting dark matter from late decays and the $H_0$ tension

We investigate a mechanism for the production of self-interacting dark matter based on WIMP-like messenger state decays into dark matter and dark radiation occurring after recombination. Such a transition leads to a mild relaxation of the Hubble tension, and at the same time may resolve the small-scale structure problems of the $\mathrm{\Lambda }$CDM. We illustrate this mechanism in the Higgs portal dark matter model, which we find to be a promising route.