"They are only in the universe" should also be acceptable, right? At least Google Translate agrees with me.
I'm not a big expert in Italian, but if it were like that, wouldn't it have been like "Sono soli/e nell'universo"? As I know, the adjetive has to agree in gender and number with the subject.
I think it's acting as an adverb instead of an adjective. Same as "they are just in the universe."