  We analyze fermion mixing in the framework of field quantization in curved
spacetime. We compute the expectation value of the energy momentum tensor of
mixed fermions on the flavor vacuum. We consider spatially flat
Friedmann-Lemaitre-Robertson-Walker metrics, and we show that the energy
momentum tensor of the flavor vacuum is diagonal and conserved. Therefore it
can be interpreted as the effective energy momentum tensor of a perfect fluid.
In particular, assuming a fixed De Sitter background, the equation of state of
the fluid is consistent with that of dust and cold dark matter. Our results
establish a new link between quantum effects and classical fluids, and indicate
that the flavor vacuum of mixed fermions may represent a new component of dark
matter.
