に違いない details

Examples

must be; surely — a confident conclusion that admits almost no doubt

この味あじ、絶対ぜったい手作てづくりに違ちがいないよ。

kono aji, zettai tezukuri ni chigainai yo.

This taste — it's got to be homemade. casual

あの慌あわてた様子ようすからすると、彼かれは何なにか隠かくしているに違ちがいない。

ano awateta yousu kara suru to, kare wa nanika kakushite iru ni chigainai.

Judging by how flustered he looked, he must be hiding something. neutral

この筆跡ひっせきは被告人ひこくにんのものに違ちがいありません。

kono hisseki wa hikokunin no mono ni chigai arimasen.

This handwriting must belong to the defendant. formal

plain clause + に違いない（N・na-adjはそのまま接続 — 元気に違いない。の・なは入らない）attaches to plain clause

Nouns and na-adjectives attach at the bare stem — 独身どくしんに違いない,
元気げんきに違いない — no の／な needed, unlike はずだ／わけだ.

はずだ
Certainty grades apart — はずだ reasons from a schedule or known fact but leaves room for reality to defect (はずだったのに), while に違いない voices near-total conviction with no such escape hatch, closer to a witness's "it has to be him" than a timetable's "should arrive".

わけがないplain-antonym
Clean antonyms at the same certainty extreme — に違いない asserts "must be" with near-total conviction, わけがない asserts "no way" with equal conviction in the opposite direction; neither hedges the way はずだ／はずがない can.