This refers to the amorphous force of darkness that seemingly emanates out from everywhere.
It is everything that is wrong with humanity: the greed, the selfishness, the cowardice. It is that which evil is built out of, and founded upon.
The "dark forces" are not so clearly defined as they are in literature and movies. They are not just the orcs, and the dark lord Sauron. They are not just the nazis, or the CEOs.
The dark forces channel through all of us. They seize the opportunity whenever we aren't on top of our game. Whenever we make the wrong choice. Whenever we take the easy way out. Whenever we shirk our responsibility to the rest of humanity, or look the other way when someone else does.
"If you choose the quick and easy path, you become an agent of evil" -Yoda