A period of entropy dilution in the early universe, in which only the standard model plasma heats up, while the dark sector plasma does not, can open up parameter space

for thermal relic dark matter models. Taking as an example fermion dark matter with dark photon mediator we find that a diluted thermal relic dark matter with mass as

light as 1 keV is phenomenologically viable. The dilution mechanism that we discuss in detail has a cosmological evolution with matter dominated period where a heavy

state gives the dominant contribution to the energy budget of the universe. The decay of the heavy state into the SM particles results in the dilution of the hidden sector.

We derive the maximum possible dilution, the constraints on the model from terrestrial experiments, astrophysics, and cosmology,

and give possible future search strategies.