This is the 6335th most frequent Korean word.
아니었을까
"Wouldn't it have been" or "Wasn't it possibly."
In this sentence, '합격하지 않았을까' implies a speculation or doubt about the possibility of passing.
그가 더 열심히 공부했다면 합격하지 않았을까 싶다.
If he had studied harder, he might have passed.
Here, '찾지 않았을까' questions an alternative possible conclusion in a hypothetical situation.
이 길이 아니라면 아마도 잃어버린 물건을 찾지 않았을까?
If not this path, might we have already found the lost item?
'가지 않았을까' speculates about what might have happened given a past condition.