Corrected version:
Space is a huge, mostly empty place with stars and planets scattered like sprinkles. It stretches so far that we call it 'boundless'.
Why we changed it (easy steps):
- The first sentence is fine. It starts with a capital letter and ends with a period.
- The second sentence needed the word that to connect the idea: It stretches so far that we call it 'boundless'. The phrase that's we call is incorrect because "that's" means "that is" and doesn't fit there.
- We put 'boundless' in quotes to show it's a special word meaning "no edges or end." You can explain it like: space goes on and on, like a playground that never stops.
Great sentence — nice comparison with sprinkles!