The term "Dark Angels" primarily refers to a fictional chapter of Space Marines in the Warhammer 40,000 universe. Known for their secrecy and mysterious past, they are one of the original Space Marine chapters created during the Great Crusade. The Dark Angels are characterized by their green armor and are dedicated to the protection of humanity, often battling against various alien threats and chaos forces. In addition to their military prowess, the Dark Angels are known for their internal struggle with a dark secret involving their fallen brethren, the Fallen Angels. This secret drives much of their actions and motivations, as they seek redemption and to atone for their past mistakes while maintaining their honor and loyalty to the Emperor of Mankind.