The word "darkness" in Ephesians 6:12 comes from a root word meaning shade or error. Another word to describe this darkness is obscurity.
Jesus said we would know the truth and the truth would set us free. The Rulers of the Darkness of this World are beings who are committed to keeping the people of this world in darkness/in error.
They accomplish this through outright lying (Satan is a liar and the father of it). They also work in more subtle ways by shading, manipulating, and obscuring the truth.
The Rulers of the Darkness of This World oppose all truth. Jesus is the Truth. His words are spirit and life. Therefore, the Rulers of the Darkness of This World are committed to opposing the truth of Jesus Christ on every level of understanding, using any method or person they can. There is an entire class of devils [fallen angels]--Powers--committed to this one thing. They are a formidable army with only one goal--that of keeping mankind in darkness concerning every truth that will save, set free, bring any blessing, healing, or relief to us whatsoever.
They bitterly hate us. And if they cannot keep us from eternal doom, they delight in causing us to suffer needlessly.
Needlessly, because so much of our suffering is needless. The scriptures say many are destroyed for lack of knowledge. The knowledge that is lacking is the knowledge revealed in the Word of God. The Truth. The Light.
The scriptures say, "Thy Word is a lamp unto my feet and a light unto my path." Jesus said, "The Blind lead the blind, and they all fall into the ditch."
We have an enemy who is committed to keeping us in the shade, to obscuring the light, blinding from the truth, and to leading as many as possible into the ditch through error.
In His Word, God has revealed how we can prevent this from happening. Commit to reading his Word daily. And then be doers of the Word, and not hearers only.