What does "Dark Funnel" mean?

Dark Funnel:

Dark Funnel" refers to the hidden or unknown factors that influence someone along the traditional "sales funnel" from someone who knows nothing about and has no interest in the product (or service), to someone who becomes interested in it--typically through an advertisement of some sort--then seeks out information about it from various sources, figures out where to get it, and procures it, becoming a customer. Traditional marketing creates consumers by creating want in prospects through ad campaigns, brand awareness, etc., eventually moving the prospect along a funnel toward purchase. The "dark funnel" is the factors out of the knowledge and control of marketers that go into someone making a purchase of their product or service.

 Shining a Light on the Dark Funnel