  We study the possibility that a generalised real scalar field minimally
coupled to gravity could explain both the galactic and the cosmological dark
components of the universe. Within the framework of Einstein's Relativity we
model static galactic halos by considering the most general action built from
the scalar field and its first derivatives. Although the gravitational
configuration is static, the scalar field may be either static, or homogeneous
and linear in time. In the case of the static scalar field, the models we look
at inevitably posses unphysical negative energies, and we are led to a sort of
no-go result. In the case of the homogeneous scalar field, on the contrary, we
find that compact objects with flat rotational curves and with the mass and the
size of a typical galaxy can be successfully modeled and the Tully-Fisher
relation recovered. We further show that the homogeneous scalar field deduced
from the galactic halo spacetimes has an action compatible with the kinetic
Unified Dark Matter models recently proposed by Scherrer. Therefore, such a
homogeneous kinetic Unified Dark Matter not only may correctly mimic the
galactic dynamics, but could also be used to model the present-day accelerated
expansion in the universe.
