Learn Italian: Nessuno può immaginare chi viva oltre questo spazio ignoto. - Nobody can imagine who lives beyond this unknown space.

This sentence contains an indirect question introduced by chi (who). Because the main verb is immaginare (to imagine), which expresses a mental process and uncertainty, the indirect question requires the subjunctive. If the main verb stated a direct fact, you would use the indicative.

What subject is viva conjugated for? I know the I, you, and he/she/it forms of the present subjunctive are identical.

It is conjugated for the third-person singular (he/she/it). In this clause, chi (who) acts as a singular subject, just as it does in English.

The sentence starts with Nessuno, but there isn't a non before the verb. Why?

When negative pronouns like nessuno (nobody) or niente (nothing) are placed before the verb as the subject, Italian does not use non. You only use the classic Italian double negative if the negative word comes after the verb, as in Non può immaginarlo nessuno.

Is ignoto just a synonym for sconosciuto?

They both mean unknown, but ignoto carries a more literary, mysterious, or absolute tone. It is often used for things that are completely unexplored or undiscovered, like outer space. Sconosciuto is the more common, everyday word for something that is simply unfamiliar.