  The so called `cosmic coincidence' problem seems to suggest a coupling
between dark energy and dark matter. A possibility could be given by dark
matter particles with masses depending exponentially on the scalar field
associated to dark energy. If the latter also has an exponential potential,
attractor solutions exist with constant Omega_{DM} and Omega_{DE} and negative
effective equation of state. Due to the very low mass of the dark energy field,
its coupling to baryons should be strongly suppressed. Here we present a
natural realization of this scenario in a model with large extra dimensions.
