  The nature of dark matter (DM) remains one of the most important unanswered
questions in particle physics. Here, we propose a novel scenario for DM in
which weakly interacting massive particles (WIMPs) can freeze-in due to a
first-order phase transition (FOPT) in the early Universe. The FOPT dilutes the
pre-existing DM density to zero and leads to a sudden change in DM mass,
preventing WIMPs from re-equilibrating due to their large mass-to-temperature
ratio. Following the FOPT, WIMPs are produced via a freeze-in process, even
though their interactions are NOT feeble. We demonstrate this concept using a
simplified model and then apply it to a realistic model with a delayed
electroweak phase transition. Our work presents a promising new direction for
the freeze-in mechanism, and also extends the category of WIMP DM.
