回答

・an unknown origin

例句
Although I inspected the broken parts, I could not find why they were broken. So, they are caused with unknown origin.
儘管我檢查了損壞的部件，但我找不到它們為什麼損壞。所以，它們是由不明原因引起的。

「原因不明的」還有其他表達方式，
"without a known cause"或"of an unknown origin" 、"from an unknown cause"
等。

有幫助｜0