The mythological element of Aether is literally what the Gods were said to breathe, as mortals breathe air. It was a more pure kind of air for the divine, and some even said that the element itself granted immortality and perhaps other dormant abilities in a being. It embodies transcendence and divinity more than any other element, even Light, and personifies both purity and the void. These two are, naturally, heavily linked.
After all, nothingness must be the epitome of purity.
The energy waves that travel through the Aether are likely unlimited in quantity, but the confirmed ones are electricity, plasma, and various energies that stars exude. Aether is the element of the stars, of outer space, of empty space, and of different dimensions. Through dimensions travel energies we have yet to discover, and Aether is indeed, for the most part, undetectable to us thus far.
Nonetheless, it's clear that without this media for energy to travel in, the universe would not be able to sustain itself. It is the fabric of creation; the threads through which everything moves. The very building bricks of the universe.
An even greater mystery are the void dimensions, in which nothing moves. They are immobile dimensions mentioned in the EtG description of Aether, and something that contradicts everything we know of the universe. Some believe they are containers to be filled with creation as energy flows through the dimensions, and some see them as the end of creation. Perhaps it is where we all return after we've dissolved into molecules of earth, water, air, and then further into space.
Into nothingness. Back to what we first were, to be used again as the universe expands. The pure essence of existence.