The Convergence is a cosmic event that occurs approximately every five thousand years. During the Convergence, all of the Nine Realms of Yggdrasil are in alignment. This alignment causes the dimensional boundaries between each realm to become thin, resulting in various physical and hyper dimensional anomalies occurring at random.
These phenomena include shifts in gravity, spatial extrusions, and the fabric of reality possibly tearing apart. Invisible wormholes that allow matter to move between realms are seemingly born and evaporate at random. As the Convergence reaches its apex, several larger, visible wormholes open in the sky above central locations in each realm, with each portal acting as a window through which parts of other realms can be seen.
During a Convergence event in the distant past, Malekith attempted to take advantage of the weakened boundaries between the realms to destroy the universe and return it to its natural state of darkness, from which the Dark Elves originated, using a powerful weapon known as the Aether. However, Malekith's attempts were thwarted by the Asgardians and their ruler Bor.
Following this, Malekith and the remaining Dark Elves retreated and entered a state of deep sleep for 5,000 years, until the time of the next Convergence, during which Malekith made a second attempt. He failed again, this time at the hand of Bor's grandson, Thor.