"unsolved question" is correct and usable in written English.
You can use it when referring to a question that has yet to be answered. For example, "The cause of the disease remains an unsolved question."
The FBI investigation remains in its early stages, but his motive is the principal unsolved question.
The large time decay for the Navier Stokes flows in L1(Rn+) is a long-standing unsolved question.
However, for hydrologists, an unsolved question must be addressed before each model application in a different target area.
But exactly how this protein-based tissue assembles itself to achieve this flexibility remained an unsolved question — until now.
The origin of the heavy r-process elements, is the biggest unsolved question in our understanding of chemical evolution in the Milky Way.
An unsolved question is whether there can exist a complete set of mutually orthogonal Latin squares of order k if k is not a prime power.
The reconstruction of a distribution knowing only a finite number of its moments is an extremely important but in practice still unsolved question for many fields of science (chemical and process engineering, electronic engineering, nuclear physics, image analysis, biotechnology…).
The island fades from view as the plot's melodramatic clichés pile up — the unsolved question of Sheilagh's own parentage, Joe's repressed love for her, the enigma of Sheilagh's mother's past.
Theoretical calculations show that this last failing is closely related to the "hierarchy problem," the name physicists give to the unsolved question of why all four forces have such vastly different and unexplained strengths.
Mr. Cordovez said at a news conference that the negotiators would go directly to the remaining unsolved question, a document containing a timetable for the withdrawal of over 100,000 Soviet troops who have been in Afghanistan since December 1979, when Mr. Karmal came to power.
The hydrolysis of hemicelluloses until today is an unsolved question [25].
