The phrase "big question" refers to an important or complicated problem or topic that requires a lot of thought and discussion to find an answer or solution. Full definition
The movie leaves lots of big questions on the table and we've collected six of the bigger - hanging plot threads. (cinemablend.com)
Nevertheless, there are undoubtedly still big question marks over their display and they'll need to produce more convincing performances moving forward. (caughtoffside.com)
This is a very big question for whose of us, likely everyone, who work in groups. (slaw.ca)