According to myth, the emptiness before creation was the domain of three mighty dragons, and everything that exists today came from these cosmic titans.
When the Progenitor Dragons, Siberys, Eberron, and Khyber fought each other, their combat stretched out across existence. After the combat, Siberys' was spread amongst the stars, while Eberron encased Khyber within.
Dragon Above: Golden Siberys, Righteousness thorough sacrifice
Dragon Between: Gentle Eberron, the fountain of life.
Dragon Below: Cruel Khyber, the master of darkness, insanity, and corruption.
Together, they held dominion over the fate of all, and jointly decided on the proper shape of the universe.
Official Eberron Wiki (Flavor is current but mechanics are out of date)
Wayfarer's Guide to Eberron - Current official 5E rules