We don't know that it would exist outside the Milky Way, even if we assume the Milky Way is more or less the same in Trek and in reality. This is never stated in "Where No Man" or the later visits to the Barrier; the Barrier is just something that stands between "staying in" and "going out", just like your front door is something standing between you and Guatemala.
Not being able to see purple haze through current telescopes is fairly irrelevant to the argument: the Trek Milky Way is not an exact
copy of the real one. But it is roughly as big, for one thing, and has similar spiral arms in certain pieces of onscreen artwork, which keeps most of the estimates of "how far is the edge" relevant even when we use the Milky Way as a yardstick. My original point was just that we don't see any purple haze outside
the real Milky Way, either, so it's a wholly fictional feature without placement limitations.
...For all we know, the Barrier defines the edge between the spiral arms and the (yes, yes, far from empty) space in between, hugging the arms tightly like a pantyhose for a starfish. And when we zoom in to the surface of the pantyhose, we see it forms a complex shape resembling the surface of a brain. Some of the folds might actually come pretty close to Earth, then (the Nexus might be a particularly mobile fold).
(Also note here that the Barrier apparently isn't a purple surface between here and there. It's something invisible
that only manifests as narrow purple shapes when you get really close to it. It wouldn't be much of a barrier if it were the shape of its visually evident part...)