Explanation
-다니까(요) repeats a statement with emphasis, as if the listener had not believed or heard it the first time: 'I said…', 'I'm telling you…'. It is the quote -다고 하다 collapsed with -니까, so verbs take -(느)ㄴ다니까, adjectives and past forms -다니까, and nouns -(이)라니까. Drop 요 for plain speech; it can sound irritated, so keep it for people you are close to. 나 진짜 안 갔다니까! — I'm telling you, I really didn't go!