  We describe the implications on the structure formation, the cosmic microwave
background (CMB) and the large scale structure (LSS) for a Universe in which a
significant part of dark matter is made of mirror baryons. Being the
microphysics of the mirror baryons identical to the one of the usual baryons,
we need only two extra thermodynamical parameters to describe our model: the
temperature of the mirror plasma (limited by the BBN) and the amount of mirror
baryonic matter. We extend the Jeans theory in order to study the structure
formation mechanism in linear regime. Using a numerical code, we compute the
temporal evolution of perturbations, the CMB anisotropies and the LSS power
spectra. We show as the decoupling time and the dissipative Silk scale of the
mirror baryons (both being different than for the ordinary ones) are key
features for the evolution of a Mirror Universe, and induces specific
signatures on the power spectra. We analyse the dependence of both CMB and LSS
spectra on the mirror sector temperature and on the amount of mirror baryons.
Confronting with the present observational data, we show that for x < 0.3 the
dark matter could be constituted entirely by mirror baryons, since in this case
mirror baryons at linear scales behave essentially like cold dark matter (CDM)
and substantial deviations are expected only at smaller scales which have
undergone non-linear growth from relatively large redshifts. In the case of x >
0.3 the pattern of the LSS power spectrum excludes the possibility of dark
matter consisting entirely of mirror baryons, but they could present as
admixture to the conventional CDM.
