  Conditions for the establishment of small density perturbations in a
self-gravitating two component fluid mixture are studied using a dynamical
system approach. It is shown that besides the existence of exponentially
growing and decaying modes, which are present for values of the perturbation
wave-number $k$ smaller than a critical value $k_{_M}$, two other, pure
oscillatory, modes exist at all scales. For $k < k_{_M}$, the growing mode
always affects both components of the fluid and not only one of them. Due to
the existence of a resonance between the baryonic and the dark perturbations,
it is shown that the onset of structure formation in the post recombination
epoch is substantially enhanced in a narrow scale band around another critical
value $k_{c}$. For dark matter particles having a mass $\sim 30$ eV, the
corresponding critical mass scale for the establishment of density
perturbations at the time of recombination is of the same order of magnitude as
the galactic one.
