I will describe how cosmological measurements can be used to test the existence of dark fifth forces. These are new long-range forces acting solely on dark matter, which lead to an effective violation of the Equivalence Principle on cosmological scales today. After discussing the main effects of dark fifth forces on cosmological observables like the CMB and matter power spectrum, I will present constraints on the strength of the new interaction as derived from Planck and Large Scale Structure data. The interplay between our bounds and searches for violations of the Equivalence Principle in the visible sector will also be outlined. Based on 2204.08484, and work in progress.