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 modi. cation 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.