Express certainty; 'it is obvious', 'it must be', 'no doubt'.
Verb-casual + に決まっている, い-Adjective + に決まっている, な-Adjective + だに決まっている, Noun + だに決まっている
Long explanation:
The ~に決まっている grammar point is used to express certainty or a strong conviction about a situation or event. It can be translated in English as 'it is obvious', 'it must be', or 'no doubt'. The formation differs depending on whether it is used with a verb, い-adjective, な-adjective, or noun.