Anima
Anima is the underlying light energy that is the source of all life within the world of Illumina. It is known by various names and is channeled by various practices around the world, with each being ignorant of the other's workings and writing them off in one way or another, when really it is all the same underlying thing. These differences in beliefs around Anima are profound, and have led to the creation of various cultures and Religions around the world. Over time these disagreements have led to wars, even. None of them truly understand how it works, though, and thus all of these methods of harnessing Anima are sub-par compared to what would be considered the "true way".
By contrast, The Darkness is an opposing force which seeks to absorb light like a sponge as much as possible.