Does Dark Matter cast a shadow?........
Dark matter doesn't do much at all. It has gravity, but it neither emits nor absorbs nor reflects light. Nor does it interact with ordinary matter except by gravity.
The shadows in the icrowave background are caused by absorption of ionized gases in these x-ray active galaxies. Microwaves can't get through these galaxy clusters because of these ionized gasses. But the microwaves will pass through galactic clusters where there is no x-ray activity and none of the ionized gasses. Perhaps the galaxies they think have ionized gas, don't.
Does it matter?