Experimental limits on any long range nongravitational interaction between dark matter and ordinary matter

Much of the mass of the Universe is thought to reside in some as yet unidentified dark matter. This view is based on the analysis of trajectories of luminous ‘‘tracers’’ that map out the local potential, assuming that gravity is the only long ranged interaction between ordinary and dark matter. This assumption should be tested experimentally if possible. Laboratory tests of the weak equivalence principle can constrain (at an interesting level) any exotic coupling between ordinary and dark matter when analyzed as a test of the uniformity of free fall towards the center of the Galaxy.