Strange quark matter as dark matter: 40 yr later, a reappraisal

Forty years ago Witten suggested that dark matter could be composed of macroscopic clusters of strange quark matter. This idea was very popular for several years, but it dropped out of fashion once lattice quantum chromodynamics calculations indicated that the confinement/deconfinement transition, at small baryonic chemical potential, is not first order, which seemed to be a crucial requirement in order to produce large clusters of quarks. Here, we revisit the conditions under which strangelets can be produced in the Early Universe. We discuss the impact of an instability in the hadronic phase separating a low density, positive-strange- charge phase from a high-density phase with a ne gativ e strange charge. This second phase can rapidly stabilize by forming colour-superconducting gaps. The strangelets then undergo partial e v aporation. In this way, we obtain distributions of their sizes in agreement with the observational constraints and we discuss the many astrophysical and cosmological implications of these objects. Finally, we examine the most promising techniques to detect this type of strangelets. We also show that strangelets can exist with masses ≲ 1017 g, while primordial black holes are ruled out in that mass range, allowing us to distinguish between these two dark matter candidates.