be
remain
These sites remain largely unknown to the British public.

completely, entirely, quite (BrE), totally, utterly (esp. AmE)
His whereabouts were quite unknown during this period.

almost, practically, virtually
This drug was practically unknown in Britain.

largely, mostly
comparatively, relatively
a comparatively unknown actor
She was then still relatively unknown.

as yet, currently, still
heretofore (formal, esp. AmE), hitherto, previously
a period of democratic development previously unknown in their country

apparently
otherwise
He cites the works of two otherwise unknown authors.

to
Gold was totally unknown to their civilization.

for parts unknown, to parts unknown
She departed for parts unknown.

for reasons unknown, for unknown reasons

Unknown is used with these nouns:
actor, actress, artist, cause, destination, fate, language, location, number, origin, planet, poet, substance, terrain, territory, whereabouts