Abstract: A dark-sector gauge symmetry implies two automatically stable dark-matter candidates: the lightest particle with a dark electric charge, and the lightest monopole with a dark magnetic charge. We consider monopole production by a thermal phase transition in the early Universe. We show that, generically, the abundance of dark monopoles is negligible with respect to that of dark particles. There is only a small window, where stable dark particles are sufficiently light for their abundance to be suppressed, yet heavy enough to satisfy constraints on dark radiation. In this case, dark matter is composed of monopoles with a mass larger than 10^8 GeV. We discuss potential observational signatures.