Description
Precision cosmology offers the possibility of testing the dynamics of the dark sector even if it is completely secluded from the visible one. As a first example, we consider the imprint of a long-range force mediated by a scalar field and acting solely between Dark Matter particles on the distribution of galaxies, also known as the large-scale structure (LSS) of the Universe. We derive the strongest constraint on such forces from a combination of Planck and BOSS data. In doing so, we consistently develop, for the first time, the Effective Field Theory of LSS in the presence of new dynamics in the dark sector. More general self-interactions of the scalar mediator can induce a dark first-order phase transition. We explore its effects on the CMB and LSS and discuss its interplay with the long-range force dynamics.