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.
