dark

两者都表示“在黑暗中”。但 in the dark 一般指经常的情况：Cats can see in the dark. 猫在黑暗中能看得见东西。

有时也指一时的情况：

All the lights went out and we were left in the dark. 所有的灯都熄灭了,我们被留在黑暗中。而 in (the) darkness 则只能指一时的情况：

1. The room was in complete darkness. 室内一片漆黑。

2. I could not see her face in the darkness. 在黑暗中我看不清她的面孔。

此外, in the dark 可用作成语, 意为“全然不知”“蒙在鼓里”。in the darkness 没有这种用法。

1. He doesn't tell me his plan, so I was completely in the dark. 他没有把他的计划告诉我，所以我完全蒙在鼓里。

pitch dark

2. We are still in the dark about the cause of the fire. 我们对于火灾的起因仍然一无所知。 pitch dark 见 pitch