Indigenous cultures can "see" dark constellations (example: the Australian emu in the sky) which are defined empty spaces which are explicitly visible.

Using this concept, one could think of or use blank index cards in a zettelkasten or even the empty (negative) spaces between cards as "dark ideas" (potential ideas which need to be thought of and filled in).

Link to: https://hypothes.is/a/FlqusEN1Ee6XEr_9StPUlA