Original: Space is called boundless because it goes on and on and we can't find an edge. Space is so big that even though it houses a lot of stars and planets it is still mostly empty.
Corrected (standard grammar): Space is called 'boundless' because it goes on and on, and we can't find an edge. Space is so big that, even though it contains many stars and planets, it is still mostly empty.
Very simple version for a 5-year-old: Space seems to go on forever. Even though it has lots of stars and planets, most of space is empty — there is a lot of room with nothing in it.