Primordial black holes from bubble collisions during a first-order phase transition

We show that collisions of true vacuum bubbles during a first-order phase transition can produce a sizable abundance of primordial black holes (PBHs), provided that bubble walls have a non-negligible thickness and the transition lasts long enough. We predict an approximately monochromatic PBH mass spectrum, and the PBHs can constitute the whole dark matter with their masses in the currently allowed range of $10^{-15}$--$10^{-11}$ solar masses if the phase transition occurs around $10^5$--$10^7\,{\rm GeV}$. Our calculation combined with bounds on the PBH abundance can constrain models with a first-order phase transition.