Strange quark matter as dark matter

Forty years after Witten's proposal, we reassess whether dark matter can be made of strangelets, macroscopic clusters of up, down and strange quarks. Using a state-of-the-art lattice QCD equation of state, we show that in standard cosmology the cosmic trajectory cannot reach the QCD critical end point, so that strangelet formation cannot rely on a first-order transition. We propose an alternative path near the crossover, combining strong quark correlations, a chemical-mechanical instability in the hadronic phase and color superconductivity. Partial evaporation then fixes the ordinary-to-dark matter ratio and a mass spectrum peaking at $A\sim10^{35}-10^{40}$. Unlike primordial black holes, strangelets do not undergo Hawking evaporation and can populate masses below $10^{17}\,\mathrm{g}$. Lunar seismometry, motivated by the unidentified Apollo shallow moonquakes, is a promising probe.