Milli-Magnetic Monopole Dark Matter and the Survival of Galactic Magnetic Fields

Dark sectors with Abelian gauge symmetries can interact with ordinary matter via kinetic mixing. In such scenarios, magnetic monopoles of a broken dark $U(1)$ will appear in our sector as confined milli-magnetically charged objects under ordinary electromagnetism. Halo ellipticity constraints are shown to significantly bound the strength of dark magnetic Coulomb monopole interactions. The bound monopole ground state, which in vacuum is stable and has no magnetic charge or moment, is shown to become quantum mechanically unstable in the presence of an external, ordinary magnetic field. If these states contribute sizably to the local dark matter density, they can extract significant energy from the galactic magnetic field if their decay occurs on a galactic timescale or less. We revise and extend this "Parker Bound" on galactic magnetic energy loss to milli-monopoles which leads to the strongest existing constraints on these states, satisfying our halo ellipticity bounds, over a wide range of monopole masses.