Does this sentence mean, literally, “To me nothing is known of his name” or “I don’t recognise his name”?
We often use in Russian the constructions withy the dative case:Мне нравится, Ему холодно, Нам ничего об этом не известно.
The meaning is here simple: I don’t know his name, I have no information about his name.
Is “неизвестно” supposed to be in one word? What if I want to say “Nothing is unknown”?
Yes, we write “не” separately most with the vebs, but together with the nouns and adjectives.
Nothing is unknown = Ничего не известно. ( because of -“ничего не”)
Но: неизвестная история, мне это неизвестно.