The Dark is Nothing
If I equate the individual to a black hole, the following question comes to mind: does the abyss contain an all-consuming fire, pulling inward all that it can, or is it empty, a void of nothingness? I would say both; in order to synthesize the two ideas, another idea might be introduced that brings a greater understanding of what seems to be a contradiction at first glance. It is the assertion that nothing isn't really nothing... that 'nothing', if it can be pointed to and called anything at all, must be 'something'.
“Strong hope is a much greater stimulant of life than any single realized joy could be.” - Friedrich Nietzsche
Hope is empty, unfulfilled, hollow. Like desire, like ambition. They are, in their own strange way, voids. And they are also an important part of the strength and reach of gravity an individual can have. They ache to fill themselves. Little ambitions, small hopes, meager embers… they allow only for a quaint field of gravity. A flame that’s fed though, a dark flame that grows into an inferno… well, I’ve been known to say that we’re all born bright and shiny, but few develop the true gravity and darkness of a Dead Star fallen in upon itself.
For the successful Dark Adept, that so-called 'nothing' is the flame within, one that grows exponentially, insatiable, devouring all it is able to, and growing more able all the while. So it is indeed 'something', but beyond the experience or range of perceptions possessed by any true servant/follower of the light, and perhaps even beyond what most people can see or grasp, in spite of whatever brushes or flirtations with it they might've had in the past. The lighter folk especially... they can't really pin it down as being anything, and so they call it nothing.
The Dark Side is a thing you must be enthralled in - in the grips of it, immersed in it, married and devoted to it - to truly perceive, appreciate, or understand. Dionysian, beyond the experience of a purely Apollonian spirituality. It is forever beyond reach of light bearers for the same reason that light will always be beyond me... I am not of it, and they are not of the Dark. To them, the dark is nothing. To us, it is everything.