  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,
$\delta$, the fluid velocity divergence, $\theta$, and an explicit expression
for the dynamics of the shear stress, $\sigma$, are obtained for a degenerate
Fermi gas in the background regime of radiation. Extensions to the dominance of
matter and to the $\Lambda$CDM cosmological background are also investigated
and lessons concerning the formation of large structures of degenerate Fermi
gas are depicted.
