  We show that a new force, which appeared in a five-dimensional generalization
of general relativity, implies precisely the flat circular velocity curves of
luminous matter observed in the outer parts of spiral galaxies. A cosmological
solution for the new force (not the galactic solution treated in this paper)
might explain the cosmic negative, or repulsive, gravitation responsible for
the accelerated expansion of the universe, attributed nowadays to ``dark
energy'' and sometimes modelled by Einstein's cosmological constant.
