I will argue that many theories in which the dark matter is a light boson (with a mass < eV) lead to theoretically and observationally interesting dark matter substructure. As a particular, directly calculable, example I will show that this is the case for any new vector boson with non-zero mass (a ‘dark photon’ or ‘Proca boson’) that is present during inflation, at which time a relic abundance is automatically produced purely from vacuum fluctuations. Due to a remarkable parametric coincidence between the size of the primordial density perturbations and the scale at which quantum pressure is relevant, a substantial fraction of the dark matter inevitably collapses into gravitationally bound solitons. The central densities of these ‘dark photon star’, or ‘Proca star’, solitons are typically a factor 10^6 larger than the local background dark matter density today. I will also mention some possible observational consequences and directions for future work.