The Outside is an endlessly expanding void. This is how you are able to travel to higher verses. There will always be space for creation.
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The Outside is essentially a void. But it does have many objects in it aside from the Omniverses. The Outside has an unknown amount of undefined spaces. Most of the entities living in these spaces believe their world is the entire world because they cannot see anything else. There are also artifical objects in the Outside brought in by advanced civilizations from the Omniverse or The Box, such as Dokaverse-based electricity generators, but once again these are very rare and may never be seen.