(Abridged) Present cosmological constraints and the absence of a direct
detection and identification of any dark matter particle candidate leave room
to the possibility that the dark sector of the Universe be actually more
complex than it is normally assumed. In particular, more than one new
fundamental particle could be responsible for the observed dark matter density
in the Universe, and possible new interactions between dark energy and dark
matter might characterize the dark sector. In the present work, we investigate
the possibility that two dark matter particles exist in nature, with identical
physical properties except for the sign of their coupling constant to dark
energy. Extending previous works on similar scenarios, we study the evolution
of the background cosmology as well as the growth of linear density
perturbations for a wide range of parameters of such model. Interestingly, our
results show how the simple assumption that dark matter particles carry a
"charge" with respect to their interaction with the dark energy field allows
for new long-range scalar forces of gravitational strength in the dark sector
without conflicting with present observations both at the background and linear
levels. Our scenario does not introduce new parameters with respect to the case
of a single dark matter species for which such strong dark interactions have
been already ruled out.Comment: 18 pages, 1 table, 9 figures. Invited paper for the special issue of
Annalen der Physik on "Dark Matter" (Ed. Matthias Bartelmann and Volker
Springel
