Here’s a hint: it has to do with commas. (You’re surprised, right?)
The mood grew dark and somber around the city as fighter ships that were not of their world silently filled the skies overhead.
I honestly have no idea why anyone would include that comma after world. I see this sort of thing a lot, though, as if the writer thought, Well, I’ve just typed several words in a row without any punctuation marks, so I’d better add a comma or something now, just in case.