"The universe is huge."
Translation:Το σύμπαν είναι τεράστιο.
It's not wrong but ο κόσμος does not necessarily refer to everything=universe, unlike the word 'cosmos' in English.
In general, ο κόσμος means the world or the people/crowd. Since the exact translation of universe is σύμπαν, no context required, I suggest sticking with that. ;)
You see, all languages have many different adjectives of this kind - describing size, appearance etc. Sometimes they may be synonymous, while in other cases there may be nuances that have to be explained. In any case, "τεράστιος" is definitely the most common word, just like "huge" is rather than "humongous", if you will. It's better to keep things simple in the exercises so that it's easier for learners to memorize the required vocabulary. Such comments can always be answered in the comments section, however.