  The information contained in galactic rotation curves is examined under a
minimal set of assumptions. If emission occurs from stable circular geodesic
orbits of a static spherically symmetric field, with information propagated to
us along null geodesics, observed rotation curves determine galactic potentials
without specific reference to any metric theory of gravity. Given the
potential, the gravitational mass can be obtained by way of an anisotropy
function of this field. The gravitational mass and anisotropy function can be
solved for simultaneously in a Newtonian limit without specifying any specific
source. This procedure, based on a minimal set of assumptions, puts very strong
constraints on any model of the "dark matter".
