And what does Dark Energy has to do with all this?
Nothing. But I'm not in the mood to recite my list right now.
The lack of shard for life/death could mean that it's not an element but considering that it's based in living (or dying) beings, a shard per se wouldn't make much sense. And that is neither evidence of life/death being an element is it could be just a.. er.. byproduct? of the interaction of the four elements.
The way I look at it, life magic is derived from things living, growing, and being happy. Death magic is derived from (for lack of a better term) suffering: the more miserable and brutal the ecology gets, the more ambient death magic there is. In short, life magic appears in "Garden of Eden" environments where the cycle of life is peaceful, largely benevolenmt, and..... (again for lack of a better word)....... fair. Death magic florishes in places that are "red in tooth and claw", where the natural order of things is brutal, competitive, unfogiving, and "unfair".