  In the very last years, cosmological models where the properties of the dark
components of the Universe - dark matter and dark energy - are accounted for by
a single "dark fluid" have drawn increasing attention and interest. Amongst
many proposals, Unified Dark Matter (UDM) cosmologies are promising candidates
as effective theories. In these models, a scalar field with a non-canonical
kinetic term in its Lagrangian mimics both the accelerated expansion of the
Universe at late times and the clustering properties of the large-scale
structure of the cosmos. However, UDM models also present peculiar behaviours,
the most interesting one being the fact that the perturbations in the
dark-matter component of the scalar field do have a non-negligible speed of
sound. This gives rise to an effective Jeans scale for the Newtonian potential,
below which the dark fluid does not cluster any more. This implies a growth of
structures fairly different from that of the concordance LCDM model. In this
paper, we demonstrate that forthcoming large-scale surveys will be able to
discriminate between viable UDM models and LCDM to a good degree of accuracy.
To this purpose, the planned Euclid satellite will be a powerful tool, since it
will provide very accurate data on galaxy clustering and the weak lensing
effect of cosmic shear. Finally, we also exploit the constraining power of the
ongoing CMB Planck experiment. Although our approach is the most conservative,
with the inclusion of only well-understood, linear dynamics, in the end we also
show what could be done if some amount of non-linear information were included.
