This source is called darkness. Darkness within darkness. The gateway to all understanding. In darkness we cannot distinguish between separate things. Darkness is nothingness.
Darkness is the absence of that.
Darkness is characterized by nothingness and what the Tao Te Ching is calling us to do is find wisdom in that nothingness. If you look at something someone has written, you look beyond just the ink on the paper. Rarely do we see the words as being made up by circles and lines on a piece of paper. We might notice the sloppiness or neatness of the handwriting, but that's it. We don't worry about it.