Afrikaans often leaves things a little more general unless you specifically want to make them definite.
Your sentence is more general. If you want to refer to a specific known set of details, sommige van die besonderhede is often more precise.
Here, onbekend is an adjective meaning unknown.
It comes after the verb is, so it is being used predicatively:
It describes the subject rather than standing directly before the noun.
In Afrikaans, adjectives often take -e when they come before a noun, but usually not when they come after a linking verb like is.
Compare:
So in your sentence, onbekend is correct because it comes after is.