n the first three quarters of the talk, I will explain how cosmology sets the strongest constraints yet on long-range dark matter self-interactions at length scales shorter than 100 kpc. In the last quarter, hopefully, I will briefly discuss the early-universe dynamics of an ultralight scalar dark matter coupled to neutrinos, focusing on the dissipative backreaction of the neutrinos on the scalar and its implications. If somehow there is time left, I will mention how the same dissipative effect affects other models.