大屠杀被认为是人类历史上的一段黑暗时期。

The Holocaust is considered a ___dark period in human history.

有些人被生活的挑战压垮，转向了社会的黑暗面。

Some people, overwhelmed by life's challenges, turn to the ___dark side of society.

他那些黑暗的思绪常常让他夜不能寐。

His ___dark thoughts often kept him awake at night.