As a kid back then, I knew that no Barrier or anything like it was thought to exist. I knew a lot of astronomy. I bought it in the story. Now I'm thinking about why.
Though non-canon obviously, the novelverse explained it as an artificial thing created by the Q Continuum to keep a similarly powerful being out of the galaxy, which also explained why contact with it gave telepathy prone people extraordinary powers as they absorbed the Qiness from it.
And yes, it would be a complete boundary, and mostly invisible.
