Dark matter, not magnetism

Battaner et al. have recently argued that interstellar magnetic fields can significantly affect the circular motion of gas in spiral galaxies and explain their observed flat rotation curves. This interpretation of galaxy kinematics challenges the conventional dark matter (DM) picture. However, the evidence for DM in galaxies is not restricted to gas motions. Measurements of velocity differences within galaxy pairs (not based on gas rotation and hence not affected by magnetic fields) imply the existence of massive haloes, whose properties are in total agreement with those derived from the DM interpretation of the observed rotation curves. On the other hand, the predictions of the magnetic-field, no-DM scenario cannot be reconciled with observations of pairwise galaxy dynamics. In this paper we discuss the above opposing views of galaxy kinematics. Finally, after giving substantial support to the conventional view that DM plays the key dynamical role on galactic scales, we turn the argument devised to test the Battaner et al. theory into a powerful tool to investigate the properties of DM haloes on scales of several hundred kpc.