The Dominarian Nexus is a system of twelve planes. It was created soon after the Brothers' War with the Sylex Blast that ended the war. The amount of energy released from this explosion created The Shard of the Twelve Worlds and crystalized the planes inside into a stable, closed system. Eleven planes, including Azoria and possibly the Nether Void, started to orbit Dominaria and lasted in this state even after destruction of the Shard itself.
The Dominarian Nexus is one of many local nexuses in the multiverse, where one plane sits in the center and the rest exist around it. In some circumstances such nexuses can come apart or accumulate additional planes, but so far only the Dominarian Nexus seems to be stable in its original form crystallized by the Shard.