Strictly speaking... it's unexplained.

If you want my personal take on it, "Fray" / your dark side is a physical manifestation of your psychological shadow archetype as a result of the soul crystal's influence (though only at the end of the 50 quest - until then s/he is either a hallucination or possessing the dead body of Fray the Dark Knight). S/he may be driven by our desires, but rather than require prayer and have his / her personality colored by that, "Fray" is a manifestation of your subconscious thoughts and desires. No mention of aetheric consumption.

The sparkle effect is actually unique; there's dark fog mixed in with the motes of light, the former of which isn't present with any primal (that I can recall).