**The
existence of dark matter in the Universe has been firmly established by
astronomical observations at very different length-scales, ranging from
the neighborhood of the Solar System to clusters of galaxies. The standard
way to notice this need for dark matter comes within the framework of mechanics:
A large fraction of the mass needed to produce the observed dynamical effects
in all these very different systems is not seen. At the galactic scale,
the problem is clearly posed: The measurements of rotation curves (tangential
velocities of objects) in galaxies show that the coplanar orbital motion
of gas in the outer parts of these galaxies keeps a more or less constant
velocity up to several luminous radii \cite{persic}. The discrepancy arises
when one applies the usual Newtonian dynamics to the observed luminous matter
and gas, since then the circular velocity should decrease as one moves outwards.
The most widely accepted explanation is that there exists an almost spherical
halo of dark matter, its nature being unknown, which surrounds the galaxy
and account for the missing mass needed to produce the flat behavior of
the rotation curves. This puzzle has stimulated the exploration of several
proposals, and very imaginative explanations have been put forward, from
exotic matter to non relativistic modifications of Newtonian dynamics \cite{milgrom}.
Because the dark matter is such that interacts very weakly with ordinary
matter, it is very difficult to detect it by other means than by their gravitational
effects on the baryonic matter. **

**Recently
we have proposed the scalar field as a candidate for dark matter in cosmos
(see the section of papers). These models consider scalar-tensor theories
of gravity where one is able to add mass terms to the total energy density
of the space-time. All modern unifying field theories models also contain
scalar fields. For example, scalar fields are fundamental fields in the
standard model of particles or in Kaluza-Klein and Superstring theories,
where scalar fields appear in a natural way after dimensional reduction.
In these theories the scalar field could be endowed with a exponential scalar
potential. Within this unifying point of view, we show in this letter a
possible model for the dark matter in the Universe, supposing that dark
matter is of a scalar field nature. **