  Cosmology offers opportunities to test Dark Matter independently of its
interactions with the Standard Model. We study the imprints of long-range
forces acting solely in the dark sector on the distribution of galaxies, the
so-called Large Scale Structure (LSS). We derive the strongest constraint on
such forces from a combination of Planck and BOSS data. Along the way we
consistently develop, for the first time, the Effective Field Theory of LSS in
the presence of new dynamics in the dark sector. We forecast that future
surveys will improve the current bound by an order of magnitude.
