Unexplained (adjective)

1. Not explained, understood or accounted for.
2. Not having a clear reason or explanation.

Origin:

Un + explained.

Examples:

1. The police are still investigating the cause of the fire, which remains unexplained.
2. The patient's sudden death is unexplained and the cause is still unknown.
3. Many people believe that there are some unexplained phenomena in the world.
4. He found it hard to understand the unexplained behavior of his friend.
5. The unexplained disappearance of the plane is still a mystery.