Satellite galaxies can be tidally disrupted as they orbit a more massive host galaxy. If dark matter (DM) experiences a stronger self-attraction than baryons, stars will preferentially gain rather than lose energy during tidal disruption leading to an enhancement in the trailing compared to the leading tidal stream. The Sgr dwarf galaxy, a satellite of our own Milky Way, is seen to have roughly equal streams, challenging models in which DM and baryons accelerate differently by more than 10%. Future observations and a better understanding of DM distribution should allow detection of DM forces only a few percent the strength of gravity.