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We propose a new alternative to the Weakly Interacting Massive Particle(WIMP) paradigm for dark matter. Rather than being determined by thermalfreeze-out, the dark matter abundance in this scenario is set by dark matterdecay, which is allowed for a limited amount of time just before theelectroweak phase transition. More specifically, we consider fermionic singletdark matter particles coupled weakly to a scalar mediator $S_3$ and toauxiliary dark sector fields, charged under the Standard Model gauge groups.Dark matter freezes out while still relativistic, so its abundance is initiallyvery large. As the Universe cools down, the scalar mediator develops a vacuumexpectation value (vev), which breaks the symmetry that stabilises dark matter.This allows dark matter to mix with charged fermions and decay. During thisepoch, the dark matter abundance is reduced to give the value observed today.Later, the SM Higgs field also develops a vev, which feeds back into the $S_3$potential and restores the dark sector symmetry. In a concrete model we showthat this "vev flip-flop" scenario is phenomenologically successful in the mostinteresting regions of its parameter space. We also comment on detectionprospects at the LHC and elsewhere.

The Vev Flip-Flop: Dark Matter Decay between Weak Scale Phase Transitions