Other forms: unexplainedly
Something that's unexplained isn't known or made clear, like an unexplained creaking sound coming from the attic of your grandparents' creepy house.
Anything without a clear reason for happening or existing is unexplained, whether it's the mysterious crack in the kitchen window or some lines of movie dialogue spoken in a language you don't understand (and without subtitles). When you explain something, you describe what it is, how it works, or why it exists — and then it's explained. Add the prefix un-, meaning "not," and you get unexplained.