Dark Halo or Bigravity?
  Observations show that about the 20% of the Universe is composed by invisible
(dark) matter (DM), for which many candidates have been proposed. In
particular, the anomalous behavior of rotational curves of galaxies (i.e. the
flattening at large distance instead of the Keplerian fall) requires that this
matter is distributed in an extended halo around the galaxy. In order to
reproduce this matter density profiles in Newtonian gravity and in cold dark
matter (CDM) paradigm (in which the DM particles are collisionless), many
ad-hoc approximations are required. The flattening of rotational curves can be
explained by a suitable modification of gravitational force in bigravity
theories, together with mirror matter model that predicts the existence of a
dark sector in which DM has the same physical properties of visible matter. As
an additional result, the Newton constant is different at distances much less
and much greater than 20 kpc.
