This film places a huge emphasis on the second Infinity Stone that we are introduced to - the Reality Stone - which manifests itself in a fluid form. 

The lore of the Aether is that a race known as the Dark Elves existed before the universe had light and created the Aether to use as a source for their power. The Dark Elves wished to exterminate all life that dawned along with the emergence of light in the universe, but the leader of the Dark Elves, Malekith, was defeated by Bor the grandfather of Thor. Only a handful of Dark Elves survived the battle and because their source of power was taken from them and hidden away, Malekith and a handful of Dark Elves retreated and went into hypersleep for millennia. 

In the Dark World, an event known as the Convergence occurs causing portals to open and link to each other (e.g Asgard to Earth, Earth to Jotunheim to Asgard etc.) Jane Foster inadvertently steps through a portal exactly to where the Aether is hidden and is infested by it. Because the Dark Elves are drawn to the power of the Aether, Malekith now knows where the Aether is and hunts Jane in order to obtain it and use its power to destroy all life in the Nine Realms connected through the Convergence. 

Needless to say, Malekith failed and the Aether was taken to Taneleer Tivan the Collector for safekeeping. Little does anyone suspect or know however that the Collector is ‘collecting’  the Infinity Stones. Ha, get it? No? He is collecting the Infinity Stones for himself and so in the end credits scene of the Dark World he says ‘1 down, 5 to go’, referring to how many Infinity Stones he still needs.