  We show that a suitable interaction between a scalar field and a matter fluid
in a spatially homogeneous and isotropic spacetime can drive the transition
from a matter dominated era to an accelerated expansion phase and
simultaneously solve the coincidence problem of our present Universe. For this
purpose we study the evolution of the energy density ratio of these two
components. We demonstrate that a stationary attractor solution is compatible
with an accelerated expansion of the Universe. We extend this study to account
for dissipation effects due to interactions in the dark matter fluid. Finally,
Type Ia supernovae and primordial nucleosynthesis data are used to constrain
the parameters of the model.
