In this talk I will show that the nonthermal nature of dark matter freeze-in production leads to large, totally correlated dark matter-photon isocurvature perturbations, which are imprinted in anisotropies of the cosmic microwave background (CMB). Isocurvature is typically expected from inflationary physics but the isocurvature from freeze-in arises post inflation. Using millicharged dark matter as a benchmark model I will discuss how to calculate the amplitude of the generated isocurvature from a Lagrangian description of the dark matter freeze-in process. I will present novel results excluding millicharged dark matter as the sole dark matter component for masses between 1 MeV - 10 GeV, using current CMB observations from Planck. I'll conclude by commenting on the broader implications of this new avenue for exploring fundamental dark matter physics.
