Speaking of the expansion of the universe, in a very distant future when the expansion speed is so high that most of the galaxies won't be visible from earth, their astronomers will be thinking the whole universe contains only a few galaxies. But wait, what if the universe we observe today also misses some parts that can't be observed anymore?

Neil deGrasse Tyson explained it more clearly: https://www.youtube.com/watch?t=436&v=TgA2y-Bgi3c

That's probably why it's called the observable universe.