Answer: unproven

AnswerCrossword Clue
UNPROVENYet to be confirmed
UNPROVENUp in the air
UNPROVENLike Fermat's Last Theorem until recently
UNPROVENNot confirmed
unprovenNot demonstrated by evidence or argument as true or existing
unproven(of a new or alternative product, system, or treatment) Not tried and tested