The point is that in English the word has plural forms.
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.
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.
IT is not a collective singular you said, please explain. It means one thing, yes? Space is collective for all the planets and things which comprise the cosmos.
Okay, usually a collective singular noun is made up of a set of potentially discrete individuals of the same class.
family = individuals of the same species
team = players on the same side
herd = (usually fourfooted) animals (usually) of the same species
furniture is an odd duck, kind of a collective, but made up of individuals of different classes (chairs, tables, beds etc) it's more a cover term like 'mammals' but for hard to determine reasons remains stubbornly non-count for native speakers.
space in the classic sense doesn't necessarily refer to celestial bodies, but the matter between them. ('outer space' might refer to celestial bodies and the matter between them and cosmos might, but the plain word 'space' even in the SF meaning doesn't. The planets and stars etc are _in_ space (like islands in an ocean) and not part of it.
I'm not a semanticist and I'm rapidly reaching the end of what I can confidently say on the subject without working out a theoretical model or doing some checking on references and I don't have the slightest intention of doing either :)
On the other hand, the very well known (in linguistics) Polish born Australian semanticist Anna Wierzbicka has written a lot on the boundaries in English between individual items and collective and substance nouns that address a lot of these issues. If you're interested look her stuff up. She's not the easiest read in the world but she's far from the worst writer in linguistics (that would be Chomsky, Noam who's writing is disorganized, opaque and aimed only at those who follow his work obsessively).