"Theorists believed" "galaxies could" - these are not words of "proof" but of speculation and hypothesis.
That's a typical scientist for you. Articles published in peer-reviewed scientific journals are full of such hedge-words. Personally, I can't think of any scientist who's going to come out and say, "This is it! Here's what's happening, I HAVE THE ANSWER!" That kind of assertion is completely unthinkable.