Here is a clear, simple correction and a short explanation.
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 contains many stars and planets, it is still mostly empty.
Corrected (simple):
We call space boundless because it goes on and on and we can't find an edge. Even though space has many stars and planets, most of it is empty.
Why this is better (for a 6-year-old):
- We removed the comma before and in the first sentence because the same thing (space) does both actions: it "goes on and on" and we "can't find an edge." That makes the sentence flow better.
- We started the second sentence with Even though and used a short comma after that little group of words. This makes the meaning clear: there are many stars and planets, but most of space is empty.
- Boundless means it keeps going and going and has no edge—there is no end we can find.
Nice work — your idea was already very good! The changes just make the sentences a little easier to read.