Baal HaSulam, The Study of the Ten Sefirot, Part 1, “Table of Questions and Answers on the Meaning of the Words“: Question 17. What is Darkness.
Behina Dalet (the fourth phase) in the Ratzon (desire), which does not receive the Ohr Elyon (the upper Light) inside her due to the Tzimtzum (restriction), is regarded as the Shoresh (root) of the darkness.
This is in spite of the fact that the Tzimtzum (restriction) in the fourth phase occurred at its own will. The fourth phase overcame its own desire to receive and began to want to be like the Creator, according to its free choice.
Of course, it cannot be said that it was completely free if it felt such horrible suffering. This wasn’t because it didn’t have the Light; it had all the Light of Infinity. But even so, it felt bad because of the reception.
Then it decided that filling itself with the Light in the fourth phase meant complete darkness. From this moment onward, this is what “darkness” means. That is, if I receive for myself from the Host, the Creator, and feel that I am the receiver; if I am enjoying not only what I’m receiving, but also the fact that I’m receiving from Him because of His love and good attitude towards me, meaning that I get both His attitude and the pleasure itself (otherwise, it would be the first phase), then I decide that it is complete darkness to me.
In reality, nothing changes. It is said that the Light is leaving, but the Light isn’t going in or out at any point. It only depends on how I treat my state, thinking that I’m in darkness.
One second ago, I was in my best state and happy about everything: the Host and the refreshments. I was getting pleasure from closeness to the Creator and could receive pleasure from everything He has. Suddenly, in the very same state, I begin to discover the contrast between us, meaning that I understand that there is another condition, equivalence of form between my qualities and His. Then, I decide that my state is complete darkness and nothing is worse than this.
From this moment onward, this decision made at a higher degree spreads over all the lower degrees, being the root of darkness in our world as well. Darkness is a feeling that I’m opposite to the Creator, that I’m the receiver, and He is the Giver.
From the 3rd part of the Daily Kabbalah Lesson 6/20/2011, Talmud Eser Sefirot