One picture carries this whole word: light leaving. Not light that has already gone — light in the act of going. The sky before rain, a theatre as the lamps go down, a screen out. The word is made for the middle of that change, which is why English keeps 'it got dark' for the plain fact and reaches for 'darken' when it wants you to watch the change happen. Two more things in the picture matter. First, it works in both directions with no change of shape: the sky can darken by itself, or clouds can darken the sky. Second, once you can see the picture, you can see why the same word is used for a colour going deeper, for hope going out of a room, and for a face closing over.
From the word’s history
The source traces 'darken' to Middle English derkenen, from dark plus the ending -en. That ending makes a verb of change out of an adjective, so dark plus -en is literally 'go dark' or 'make dark'. The modern core is still exactly that, which is why the word insists on the change rather than the state.
來源記載 darken 來自中古英文 derkenen，也就是 dark（暗的）加上字尾 -en。-en 的作用是把形容詞變成「變成……／使變成……」的動詞。所以這個字從一開始就不是在講「暗」這個狀態，而是在講「往暗的方向去」這個過程。今天的核心畫面正好還是這樣。
Wiktionary entry for darken, etymology_text supplied in this packet (kaikki:3f6f69a190036ae4:569092371)
登入就可以看每一支怎麼連回核心畫面。