The -다 ending is one of the common sentence ending forms. It is not the infinitive form (infinitive forms are very rarely used).
Compare the ending forms for verbs and adjectives (되다(v) and 좋다(a) as examples).
* -ㅂ니다: formal and polite (to someone older, in public speech). 됩니다, 좋습니다.
* -다: impersonal and neutral/casual (in dry written material, between close friends, to children). 된다, 좋다.
* -아/어요: social, polite (default form between adults in everyday settings). 돼요 (from 되어요), 좋아요.
* -아/어: informal, familiar (between close friends). 돼, 좋아.
(For the present tense, the adjective ending(좋다 in this case) becomes the same as the root form. A verb in the present tense (된다) is not the same as the root form (되다). This is one of important differences in verb/adjective usage.)
The sentence endings don't change the meaning. They only change the politeness, formality, and nuance.
The -다 ending can have these nuances relative to the more common -아/어(요) endings.
1. surprise or enthusiasm as in an exclamation.
2. giving an objective fact (drawing attention to it).
3. talking to oneself (there are better forms for this though).
A: 뭐 찾아? (-아/어 ending sounds the most normal with personal and social feel)
B: 응, 내 차 키가 안 보여서 (찾고 있어).
A: 잘 찾아 봐. 어디 있겠지.
B: 아! 찾았다! 탁자 옆에 떨어져 있었네. (-다 here is more of an exclamation (to both oneself and the listener) than part of the dialogue. 찾았어 would retain the normal calm tone and won't carry the same kind of surprise.)