We want to know the truth, but he kept it dark.我们想要知道实情，他却守口如瓶。

keep dark about sth. 相当于 keep secret，保守秘密。
　　再如，
　　Peter told me when he was to get married，and I promised to keep it dark．
　　彼得告诉我他结婚的时间，我答应为他保密。