As long as darkness exists (which also requires a space that can be dark to exist) then it is not true that "everything does not exist", so it's not the situation we are trying to describe.
And I'm not speaking of semantics. I'm speaking of objects and other things. And events, actions, states, characteristics etc.
As long as you understand the difference between void (the 'empty' space within the Universe) and nothing, you should understand why nothing cannot be dark.
Basically, if you want to check if a sample is something or nothing, you might simply check if it has any characteristic. If it is empty for example, it IS empty, and therefore it IS. And thus it is something and not nothing.
If it is dark then it also IS. If it is bright, or even if it is neither bright nor dark. It IS.
If the sample cannot be described as "bright", "dark", "both bright and dark", "neither bright nor dark", nor any in between... then the very fact it cannot be described is a characteristic so it IS.
Well, in fact not having any characteristics is a characteristic, which causes a paradox
Though it's still something.
Thus any sample in fact proves to be something.
So, generally trying to describe or define 'nothing' is simply unreasonable, because if it can be defined (or if cannot) then it's not 'nothing'.
Anyway, when I think about it, I begin to understand why the understanding of the true concept of 'nothing' is such a hard task that mortals are incapable of. And why those using it as a weapon (though when you use 'nothing' as a weapon, you actually don't use anything) are so powerful. And why it looks so awesome and sparkly