  We investigate the complex interactions between the stellar disc and the
dark-matter halo during bar formation and evolution using N-body simulations
with fine temporal resolution and optimally chosen spatial resolution. We find
that the forming stellar bar traps dark matter in the vicinity of the stellar
bar into bar-supporting orbits. We call this feature the shadow bar. The shadow
bar modifies both the location and magnitude of the angular momentum transfer
between the disc and dark matter halo and adds 10 per cent to the mass of the
stellar bar over 4 Gyr. The shadow bar is potentially observable by its density
and velocity signature in spheroid stars and by direct dark matter detection
experiments. Numerical tests demonstrate that the shadow bar can diminish the
rate of angular momentum transport from the bar to the dark matter halo by more
than a factor of three over the rate predicted by dynamical friction with an
untrapped dark halo, and thus provides a possible physical explanation for the
observed prevalence of fast bars in nature.
