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.