That movie is set in outer spaces."
Not in everyday, mainstream usage it doesn't. Specialist vocabulary takes on different meanings and can generate new forms that aren't part of mainstream usage.
I never claimed the plural usage is mainstream. There are an estimated 900,000 word in the English language. A vast majority of them are not in mainstream use. They still exists as valid words. The same hold true for "cosmic spaces". Not your everyday concept, but it is in use.
If you go around speaking to non-specialists saying things like:
"That movie is set in outer spaces."or "That new telescope can see further out into the cosmoses than ever before."
People will understand you, but you'll sound weird and the great majority of native speakers (let's say 98% give or take a few points) won't produce those sequences naturally.
I wouldn't worry about people's reactions. In this day and age, in the so called civilized countries there are still people who do not understand why hands should be washed before meals, or what the value is.
It doesn't have a plural forum as I'm talking about Space and not a space. A space (1) of course can be pluralised to 2 spaces.
Kinda like God and gods? To some you cannot pluralize the word "god" as they are convinced there is only one. Point of view, philosophy and tradition are one thing, linguistic reality is another. There is a significant body of work where the term is used on a regular basis. Again, pluralized space in reference to cosmos is not your average household concept, but the concept exists and is very much alive.