New limits on light dark matter - proton cross section from the cosmic large-scale structure

We set the strongest limits to-date on the velocity-independent dark matter (DM) - proton cross section $\sigma$ for DM masses $m=10\mathrm{k}\mathrm{e}\mathrm{V}$ to $100\mathrm{G}\mathrm{e}\mathrm{V}$, using large-scale structure traced by the Lyman-alpha forest: e.g., a 95% lower limit $\sigma <6\times {10}^{-30}{\mathrm{c}\mathrm{m}}^2$, for $m=100\mathrm{k}\mathrm{e}\mathrm{V}$. Our results complement direct detection, which has limited sensitivity to sub-GeV DM. We use an emulator of cosmological simulations, combined with data from the smallest cosmological scales used to-date, to model and search for the imprint of primordial DM-proton collisions. Cosmological bounds are improved by up to a factor of 25.