The fact that some galaxies have shadows and some don't doesn't go to contention that the microwave radiation is proof of the Big Bang. The radiation is there.
What we apparently are lacking is an understanding as to why some galaxies don't shield it.
That's possible, but it's also possible that the cosmic microwave background is a different or non-uniform age...which would explain why some shadows are seen but other expected shadows are not, though would be problematic for the Big Bang theory to explain.
Because those galaxies are lacking, for whatever reason, the elements/materials that absorb that particular radiation?
OTOH, maybe the materials route the radiation around/through the galaxy?
After all, didn't somebody last year came up with a material (copper-based, iirc) to route microwaves around an object?