  Light feebly interacting dark matter is widely predicted in a plethora of new
physics models. However, due to very feeble couplings with the Standard Model
particles, its relic density produced via the relativistic thermal freeze-out
process easily exceeds the observed value. The entropy dilution in an early
matter-dominated era provides an attractive mechanism for solving such an
overabundance problem. In this work, we note that this dark matter dilution
mechanism will lead to two distinctive kinks in the primordial GW spectrum,
whose frequencies strongly correlate with the DM mass. We show that the GW
detectors, such as Cosmic Explorer (CE) and Big Bang Observer (BBO), can
measure the kinks in the primordial GW spectrum and will offer a new avenue to
probe light feebly interacting dark matter.
