It's not a thing...but a quality, persuasion, even philosophy and way of life. It exists with the Light, ever intertwined. As I understand it, the Dark Side favors power. The Dark is a constant shadow to the Light, ever attempting to seduce Force users to fall to its path.
The Light Side requires focus, serenity, peace. The Dark Side is quite the opposite: anger, rage, and passion fuel it.
Private The others will quote empty platitudes to you. Ask me what you will of the Dark side and I'll truthfully speak from experience.