Because distant celestial objects accelerate a lot of matter away from them as parts of different processes. For example if there were a sizeable chunk of space were antimatter than at the edge of this space there would be a constant glow of annihilation. We don't see that anywhere (well except things like quasars doing this at small scales.
replySo, yea, it would have to be outside the observable universe in which that means we can do no empirical science on it therefore it not being real to us.
What if most galaxies are actually dark matter, and we are the exception?
replyDepends exactly how your phrasing this.
replyDark matter represents far more of the universe than 'light' matter by volume if our physics is correct, so by that most galaxies are mostly dark matter by default. We have found a few very small ones that don't seem to be.
With this said dark matter still influences the curvature of space time so its not completely invisible. We could see a dark galaxy because it would alter the images of galaxies behind it with things like Einstein rings and crosses.