It might help to think about what you want your fictional universe to hold and how large it is compared to other universes or worlds. [substeps] Maybe the universe is the larger world, as far as your characters know, and there are several planets or lands within the universe.
[header] How to create a fictional universe [title] Consider the scope of the universe. [step] One of the first elements of the universe you should focus on is the scope of the universe, or how much physical space the universe takes up in a larger world. Consider how big you want your universe to be.