The Dark Elves are the oldest known species in existence, hailing from the obscure realm of Svartalfheim. They are mortal enemies of the Asgardians. Some members of their race have been shown to have the fortitude to wield the power of an Infinity Stone. They are now presumed to be critically endangered as a species or even extinct.
The Dark Elf race originated in the Ginnungagap, the vast, primordial darkness that existed prior to the creation of the manifest universe, where they reigned absolute and unchallenged. When the universe came into being and the other civilizations of the Nine Realms arose, the Dark Elves sought to return the universe to its former state of darkness.
It was then that their leader, the sorcerer Malekith, engineered the Aether, a super weapon forged from the substance permeating and surrounding the universe before it came into being.
The Asgardians, led by their then-king Bor, fought a fierce war with the Dark Elves, eventually driving them back to Svartalfheim as Malekith attempted to use the Aether. In that final battle, with the Convergence approaching, the Dark Elves fought with all of their might, utilizing their advanced technology as well as their elite troops, the Kursed, to attempt to halt the Asgardian advance.
The Asgardians were able to triumph, however, launching an attack on Malekith and his bodyguards. During the struggle the Aether was lost to Malekith, taken by the Asgardians via the Bifrost. His plans ruined, Malekith launched a final, suicidal attack on the Asgardians, dropping the Dark Elf ships out of the sky in an effort to crush all beneath them.
Malekith himself escaped the battle, along with dozens of his kin and his lieutenant, Algrim, who would later become one of the Kursed himself. The battle caused the utter destruction of the rest of the Dark Elves, leaving those that escaped aboard Malekith's ship as the only survivors.
Five thousand years later, as the Convergence neared again, Malekith attempted to use the Aether again. By this point, however, it had planted itself within the body of the human woman Jane Foster. Malekith led his kin in an attack on Asgard, where Thor had taken Foster.
Using cloaking devices, Malekith's ship, along with a number of smaller ships, were able to launch a surprise attack on Asgard, using their nimbleness and durability to smash through the Asgardian defenses, though two of the ships were destroyed by the Asgardians in the process. The final ship, with assistance from Algrim, now the last of the Kursed, managed to break through the Asgardian defenses and disgorge its complement of Dark Elf soldiers, along with Malekith himself.
The Dark Elves engaged the Asgardian soldiers, both sides suffering heavy casualties as Malekith made for the chambers of Frigga. The Dark Elves were eventually driven away, bereft of their prize, losing three ships and dozens of soldiers in the process.
When Thor, Foster, and Loki came to Svartalfheim in order to trick Malekith into removing the Aether from Foster where it would be destroyed, a number of Dark Elves accompanied Malekith and Algrim on to the field. In the ensuing fight, Loki and Thor killed all of the present Dark Elves, including Algrim, though seemingly at the cost of Loki's life. The rest went to Earth, where they could disperse the Aether to all nine realms and destroy the universe. The rest of the Dark Elves were killed in the ensuing battle, along with Malekith.