  Our present understanding of the universe requires the existence of dark
matter and dark energy. We describe here a natural mechanism that could make
exotic dark matter and possibly dark energy unnecessary. Graviton-graviton
interactions increase the gravitational binding of matter. This increase, for
large massive systems such as galaxies, may be large enough to make exotic dark
matter superfluous. Within a weak field approximation we compute the effect on
the rotation curves of galaxies and find the correct magnitude and distribution
without need for arbitrary parameters or additional exotic particles. The
Tully-Fisher relation also emerges naturally from this framework. The
computations are further applied to galaxy clusters.
