In fact, the appearance must be in good part dramatic license: while deflectors say there's something there, sensors say there isn't. They don't radiate in any scheme the sensors routinely detect: Spock explicitly denies this. And they have either no density and no energy at all, or else their density and energy have negative values (Spock's statement there is unfortunately ambiguous; some interesting cosmological effects exist if there is a meaningful negative energy).
They wouldn't necessarily form a noticeable barrier to extragalactic astronomy either, provided the dimming and blurring of light were a stable and reasonably uniform phenomenon. (And provided it's out there already: it might not have formed yet. And provided that it's a real-space phenomenon rather than something obvious in subspace, which we have no known methods of detecting at present for good reason.) We'd just estimate the universe to be larger than it actually is, as extragalactic bodies would be dimmer than would appear without the barrier.
The ``atmospheric'' effects of turbulence within the barrier, if it blurs or distorts light coming from outside the galaxy, would not necessarily stand out: while atmospheric turbulence is a nuisance for telescopes on the Earth looking up, it's not as urgent a problem for telescopes in orbit looking down, where the atmosphere is more like a slender largely-translucent phenomenon at the focal length.
I'm suspicious of the barrier at the edge of the galaxy, but I can't say offhand that it's an utterly impossible thing.
