The hypothesis of a tiny fraction of the cosmic inventory evolving cosmologically as a degenerate Fermi gas test fluid at some dominant cosmological background is investigated. Our analytical results allow for performing preliminary computations to the evolution of perturbations for relativistic and non-relativistic test fluids. The density fluctuation, δ, the fluid velocity divergence, θ, and an explicit expression for the dynamics of the shear stress, σ, are obtained for a degenerate Fermi

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... in the background regime of radiation. Extensions to the dominance of matter and to the ΛCDM cosmological background are also investigated and lessons concerning the formation of large structures of degenerate Fermi gas are depicted.