In Thor: The Dark World Odin explains the origin of the aether as
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. Malekith sought to use the Aether’s power to return the universe to one of darkness. But, after eternities of bloodshed, my father Bor finally triumphed; ushering in a peace that lasted thousands of years.
I didn't think that Malekith was a titan/an eternal - as such, how can someone like him posses the power to create an Infinity Stone?