If something is unexplained, nobody has given a reason for it — so it stays strange. Notice carefully what the word actually says: it is not that the thing cannot be understood, it is that no explanation has been offered or found. The mystery lives in the missing reason, not in the thing itself. This is why the word turns up so often in news reports, police reports, and hospital notes, where people are being careful not to guess: 'an unexplained death', 'unexplained delays', 'unexplained weight loss', 'unexplained lights in the sky'. In all of these, the writer is saying 'we do not know why, and we are not pretending to.' That careful, official flavour is worth feeling. 'Unexplained' is a slightly formal word. In everyday talk a Taiwanese student would more likely hear 'nobody knows why' or 'for some reason'. But in writing — and in the exam reading you will meet — 'unexplained' is everywhere. Two useful patterns: 'remains unexplained' (the reason still has not been found, often after a long time) and 'for unexplained reasons' (a slightly dry, sometimes faintly annoyed way of saying something happened and nobody told us why). It goes before a noun or after 'be'/'remain'. There is also a small honest wrinkle: doctors especially use it to mean 'we have tested and found no cause', which sounds neutral but often worries patients — the word carries a quiet suggestion that someone has already looked.