Same way as the pressure only pushes "inward". The sign is reversed.
If there is vacuum everywhere:
vacuum --- us --- vacuum --- galaxy --- vacuum
Why would galaxy go away from us? I expect that the vacuum on the left of the galaxy applies the same force on it as the vacuum on its right and thus should have a null net effect on its movement?
Using your same diagram
vacuum <---- us -----> <----vacuum----> <-----galaxy---->