I hate to shoot down the shielding theory, but the force woule be like a gas around BB's, acting uniformly. It can't come from outside without being here too because we've not just had seconds, minutes, or hours to reach equilibrium with it, but billions of years. If it came from elsewhere we'd have noticed that distant objects act differently elsewhere, and we haven't.
Expansion pressure sounds like light pressure, one of the ideas Newton rejected, and he wore tube socks and funny pants. It's a notion that always get rejected but a notion that alway occurs, even to people in tube socks.