[ike77]

I don't get it?

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?

[cambalache]

Imagine that space (well space-time) it is a single elastic band and you pull it by both ends, you will have a more or less uniform tension in the middle part of the band and yet everything is receding from you. Same principle here.

Using your same diagram

vacuum <---- us -----> <----vacuum----> <-----galaxy---->