The cause of the 'great barrier' is a bit dodgy, at least by physics as we currently understand it. Any love out there for the idea that it is some sort of artificial creation? An intentional hindrance to some threat or nuisance (probably the latter, relative to anyone who could cobble up such a 'space fence')? If it is artificial, then the Milky Way becomes either a sort of jail or protectorate.
Though it does seem so as visually depicted, is the barrier just at the galactic disc perimeter? I'll assume it's all encompassing, as no reference to hopping up around it has ever been made. You'd think such a detour would be relatively insignificant on an intergalactic jaunt.