Zhè shì de yuányīn dàobùmíng, zhǐ néng mànmàn chá.
The cause of this matter can't be pinned down — we can only investigate slowly.
Tips
usage
Almost always appears in the fixed pair 说不清道不明 (shuōbuqīng dàobùmíng) — 'can't speak clearly, can't explain' — used for ambiguous feelings, office politics, unresolved relationships.