Unanswered question
Just like it sounds: it's a question that has not yet been answered
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An “unanswered question” is just what it sounds like: a question that does not yet have an answer. You can use this phrase to highlight things that are still unknown.
There are still several unanswered questions about the whale that was discovered in Peru. First, what did the whale eat? Second, where else did it live (besides where it was discovered)? Third, when did it go extinct?
Since the species was just discovered, and since there were only a few bones found, there are still a lot of unanswered questions.
Use this phrase at the beginning of any investigation or research. At the beginning of an investigation, police might say there are still a lot of unanswered questions about how a crime occurred.