Malekith, the leader of the Dark Elves, created a weapon out of the Reality Stone called the Aether. It is known to be able to convert matter into dark matter, and otherwise maintains an almost lifelike existence that causes it to bond in a parasitic fashion to a living host.

How did Malekith get the Aether?

Malekith absorbs the Aether Sensing the Aether's presence on Asgard, Malekith's lieutenant, Algrim, became the last of the Kursed warriors, allowing himself to be captured by the Einherjar. ... Through Frigga's sacrifice, Malekith and his army left before he could obtain the Aether.

How was the Aether created?

Their leader, Malekith, made a weapon out of that darkness, it was called the Aether. While the other relics often appear as stones, the Aether is fluid, and ever changing. It changes matter into dark matter, and seeks out host bodies, drawing strength from their life force.