。
It's getting dark outside.
Literal
Outside [subject-が] become-dark-[come-てきた].
A very common sentence pattern: [adjective in -く form] + なってくる 'has come to be X' where ~てくる marks the change as moving into the speaker's awareness over time. 暗くなる 'to become dark' uses the ~く continuous form of 暗い + なる. Here てきた (past of てくる) captures the gradual, ongoing transition from light to dark that the speaker has just noticed.