  The growth rate of matter perturbations is computed in a magnetized
environment for the LambdaCDM and wCDM paradigms. It is argued that the baryons
do not necessarily follow into the dark matter potential wells after they are
released from the drag of the photons. The baryonic evolution equations inherit
a forcing term whose explicit form depends on the plasma description and can be
deduced, for instance, in the resistive magnetohydrodynamical approximation.
After deriving an analytical expression for the growth rate applicable when
dark energy does not cluster, the effects of relativistic corrections and of
the inhomogeneities associated with the other species of the plasma are taken
into account numerically. The spectral amplitudes and slopes of the stochastic
magnetic background are selected to avoid appreciable distortions in the
measured temperature and polarization anisotropies of the Cosmic Microwave
Background. The growth of structures in the current paradigms of structure
formation represents a complementary probe of large-scale magnetism in the same
way as the shape of the growth factor and the associated indices can be used,
in the conventional lore, to discriminate between competing scenarios of dark
energy or even to distinguish different models of gravity.
