Wikipedia
Da'at or Daas (" Knowledge", Hebrew: דעת ) is a Hebrew word that means Knowledge. In the branch of Jewish mysticism known as Kabbalah, Da'at is the location (the mystical state) where all ten sephirot in the Tree of Life are united as one.
In Da'at, all sephirot exist in their perfected state of infinite sharing. The three sephirot of the left column that would receive and conceal the Divine light, instead share and reveal it. Since all sephirot radiate infinite self-giving Divine Light, it is no longer possible to distinguish one sephira from another, thus they are one.
Da'at is not always depicted in representations of the sefirot, and could in a sense be considered an "empty slot" into which the germ of any other sefirot can be placed. Properly, the Divine Light is always shining, but not all humans can see it.
The concealment or revelation of the Divine Light shining through Da'at does not actually happen in Da'at itself. It only seems that way from the human perspective within Malkuth. The perception of change can only occur in Malkuth. Humans who become self-giving ( Altruism) like the Light become able to see it, and for them the benefits of Da'at's light seem "revealed". However, humans who remain selfish ( Selfishness) cannot see it, and for them its benefits seem "hidden".