Velocities as a probe of dark sector interactions

Dark energy in General Relativity is typically non-interacting with other matter. However, it is possible that the dark energy interacts with the darkmatter, and in this case, the dark matter canviolate the universality of free fall (the weak equivalence principle). We show that someforms of the dark sector interaction do not violate weak equivalence. For those interactions that do violate weak equivalence, there are no available laboratory experiments to probe this violation for dark matter. But cosmology provides a test for violations of the equivalence principle between darkmatter and baryons - via a test for consistency of the observed galaxy velocities with the Euler equation.