나 보다

must be; I guess

Draws a conclusion from a clue rather than direct knowledge: everyone must be asleep since it's late, it must have been rush hour, the teacher seems to be getting on in years, there must be chili powder in it. It is used about other people and about situations you can only infer, not about your own feelings, which you already know — the one self-directed example is about a past moment the speaker can no longer account for. It is close to 는 것 같다 but leans harder on the evidence, landing as "that would explain it".

Register: spoken; conversational