  Distance measurements to type Ia supernovae (SNe Ia) indicate that the
universe is accelerating and it is spatially flat. Otherwise, the rotation
curves for galaxies or galaxy clusters reveal the existence of a dark matter
component. Then, approximately 0.35 of the total energy density of the universe
consists of dark matter, and 0.65 of the energy density corresponds to dark
energy. These two dark components of the universe have different properties
such that: dark matter clusters gravitationally at galactic scales while dark
energy, dominating at large scale, does not cluster gravitationally. It seems
difficult to find a candidate which explains the observations at both large and
small scales. In this paper we take into account a scalar field, more precisely
a rolling tachyon arising from string theory, which seems be capable to explain
the observations at different scales. We examine this scalar field behavior at
galactic scales and we fit the rotational curve data of spiral galaxies. Our
results are comparable with the curves obtained on one hand from a complex
scalar field, which only plays the role of the dark matter at galactic scales,
and on the other hand from the MOND model.
