It must be something completely known, but put into a contrasting setting.

The cause of this transformation, in turn, is not completely known, however.

The severity of his injuries was not completely known until he returned to Europe months later.

The train of events leading up to the coup may never be completely known.

The biological systems are themselves still not completely known at this scale.

After the war, both countries concluded their investigations, not completely known to public.

The hands, feet and tail, including the club, are therefore not completely known.

The full environmental consequences of the catastrophe are not yet completely known.

The origin and meaning of this word are not completely known.