Explanation
A strong rhetorical question: 'If not X, then what is it?'
Formation
Noun + でなくてなんだろう
Detailed explanation
「Noun + でなくてなんだろう」expresses the speaker’s strong conviction that something must be that noun—no other possibility. Literally, 'If it isn't Noun, then what is it?'
Examples
これが真実でなくてなんだろう。
Kore ga shinjitsu de nakute nandarou.
If this isn’t the truth, then what is?
彼がリーダーでなくてなんだろう。
Kare ga riidaa de nakute nandarou.
If he isn’t the leader, then who is?
これが愛でなくてなんだろう。
Kore ga ai de nakute nandarou.
If this isn’t love, then what is?
これが最高のレストランでなくてなんだろう。
Kore ga saikou no resutoran de nakute nandarou.
If this isn’t the best restaurant, then what is?
Practice
Grammar content sourced from hanabira.org.