A lower bound on the mass of dark matter

I will argue that if the density of dark matter in the early universe is dominated by subhorizon, non-relativistic field modes, then there is a relatively model-independent bound on the mass of dark matter particles [m > 10^(-18) eV]. The bound comes from considerations of (observed lack of) suppression of structure due to free streaming, and (observerd lack of) an enhancement due to a isocurvature white noise contribution. Additional assumptions relevant for some well-explored models can strengthen the bound significantly. The talk will be based partly on [https://arxiv.org/abs/2211.09775], and ongoing work. Time permitting, I will also discuss some novel phenomenology of ultralight bosonic dark matter with different intrinsic spin.