a question mark
COMMON
1. If there is a question mark over something, people do not know if it will continue to exist or what it will be like. There is a question mark over the future of old people's care as a result of the government cuts. Note: You can also say that there is a question mark hanging over something. There is a question mark hanging over the England manager's job. Note: Question mark is used in many other structures with a similar meaning. There's going to be a question mark about future US aid.
2. If there is a question mark over something, people are not sure if its quality is good. There is a question mark over the vehicle's long-term reliability. Note: You can also say that there is a question mark hanging over something. I am very pleased they have been acquitted. However it leaves a big question mark hanging over the original trial. Note: Question mark is used in many other structures with a similar meaning. There will be a question mark on his record that might stop him being promoted in the future.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012