Long before the birth of light there was darkness, and from that darkness came the Dark Elves. Millennia ago the most ruthless of their kind, Malekith, sought to transform our universe back into one of eternal night. Such evil was possible through the power of the Aether, an ancient force of infinite destruction.
~ Odin on the Dark Elves
The Dark Elves or Svartálfar (also less commonly known as Älfeneel or Harudheeniksel) are major antagonists in the 2013 Marvel film Thor: The Dark World. They are an ancient race from the realm of Svartalfheim and are one of the oldest known races in the entire universe and are one of the sworn enemies of the Asgardians. They seek to acquire the Aether (the Reality Stone) and want to return the universe back into its former state of darkness.
Thor: The Dark World
The Dark Elves are a proud and ancient race having been around centuries before the "Great Beginning" (Big Bang). Firsts of their kind came to existence at the Ginnungagap ("yawning abyss"), the vast, primordial darkness that existed prior to the creation of the manifest universe before making Svartálfheim, a world that is almost completely corroded with dark matter and one of Nine Realms, as their home. Dark Elves' connection to Dark Energy allowed them to survive harsh environment of their homeworld and for a time, they reigned absolute. When other races and civilizations began to arise across the Nine Realms, the Dark Elves waged war with the other races and sought to return the universe back into its former state of darkness. It was then that the Dark Elf Leader Malekith, created a superweapon made from the substance that existed in the universe prior to its existence and called it the Aether.
The Asgardians, lead by the then-king Bor, waged a fierce war against the Dark Elves eventually driving them back to their homeoworld of Svartalfheim. In the final battle, Malekith attempted to use the Aether on the eve of the Convergence and the Dark Elves sent out all of their troops from the Kursed Soldiers to their advanced technology to slow down the Asgardian advance. Despite their best efforts however, the Dark Elves were defeated and the Asgardians triumphed and Dark Elf ships were crashed down in a last attempt suicide attack by orders of Malekith after the Aether was taken from him by the Asgardians via the Bilfrost. Malekith, his lieutenant Kurse and many other Dark Elves managed to survive and escaped from the battle heading towards an Ark starship. The Dark Elves went into hibernation as the Asgardians withdrew and the rest of the Dark Elf species was killed in the battle.
Following the deaths of Malekith and the Dark Elves, the Aether was turned over to the Collector for safekeeping, as the Asgardians know that having all six Infinity Stones together would be disastrous. Needless to say, the Collector is planning to collect all of them.
Guardians of the Galaxy
In the film, a live Dark Elf specimen was found in the possession of the Collector, implying that he survived his race's extinction before being captured by the Collector. Just as the Collector is about to pay the Guardians for delivering the Orb (the Power Stone) to him, the Collector's assistant Carina attempted to use the Orb to free herself from servitude, but the power of the Orb only killed all of the Collector's captives and servants, including Carina and the Dark Elf.
As of currently, the Dark Elves are now considered extinct.
Though they don't make any physical appearance, the Dark Elves are mentioned by a drunken Thor as he goes over the events of Thor: The Dark World while telling the Avengers about his knowledge of the Aether/the Reality Stone. Later on, as part of the Avenger's plan to undo Thanos' actions, Thor and Rocket Raccoon go back in time to Asgard to before Malekith's attack to get the Aether from Jane Foster. While there, Thor tries to warn his mother Frigga of her impending death at the hands of Malekith and Kurse, though she refuses to hear about her own future.