  The non-abelian symmetry of a lagrangian invalidates the principle of
superposition for the field described by this lagrangian. A consequence in QCD
is that non-linear effects occur, resulting in the quark-quark linear potential
that explains the quark confinement, the quarkonia spectra or the Regge
trajectories. Following a parallel between QCD and gravitation, we suggest that
these non-linear effects should create an additional logarithmic potential in
the classical newtonian description of gravity. The modified potential may
account for the rotation curve of galaxies and other problems, without
requiring dark matter.
