It is Garganta (and not outer space) what exist between the "Worlds".
The Garganta is not outer space, and it doesn't contain celestial bodies, it is a "void" filled with a chaos of swirling Reishi:
If anything, Gargata itself can not have celestial bodies, but it can only have "pockets" or "bubles" of space, and are these the ones that may contain celestial bodies.
Yet, each "World" have their own stars and outer space:
To say that the Realms are just Planets, we would fall in 1 of 2 possible traps:
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Either, we accept that the Garganta is between the worlds, separating them, and deny the fact that outer space exist, and that each of this worlds have outer space with stars. Dennying any canon depiction or mention of outer space or stars.
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Or, we denny the existence of Garganta, and say they are both in the same floating in the Same Outer Space:
That leaves us with this as the most logical configuration based on canon depictions and statements:
Furthermore, Garganta seems to be based on the idea and concept of Hyperspace:
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Garganta --> depicted as dark mist, described as swirling.
3.
Hyperspace --> described as a place where "bubbles of space" form
Garganta --> described as a place where "bubbles of space" form
4.
Hyperspace --> described as a dimension that separates (parallel universes --> synonym of --> parallel worlds)
Garganta --> a described as a dimension that separates parallel worlds (often, synonym of ---> parallel universes)
Which again, is a Higher dimension that separates Parallel Universes:
The World Of The Living seems to be based on our own world by itself alone.
As such, it is only natural to think that Bleach's Earth would exist in a Universe just like ours.
And that would have to be the World Of The Living.
Soul Society been a dimension with it's own outer space, and stars, and been called a "Parallel World" and been separated by Garganta only leave us to think of something.
In Bleach, they call the whole entirety of all things as "The Universe". But given the evidence, it may seem like The "Universe" may in fact be made of more than one "universe", in the conventional sense of the word.
Not only this is what makes most sense, based on all canon depictions and statements.
But also.
It is what makes sense according certain general ideas:
Kaname's friend loved star gazing at night. Watching the stars made her happy.
She stated that for her, been a Soul Reaper was equivalent to been given the right to literally protect the stars she sees in the skys.
This is in line and competely matches the idea that if the worlds get destroyed, so do the stars, which is again a perfect match with the structure and configuration we could observe based on canon depictions and descriptions.