I think it is safe to say that for all of our aquired knowledge, there is more that we do not know than that which we do know. If this is indeed true, It follows then that the bulk of all which can be known lies within the realm of that which we do not know that we do not know - that dark body of knowledge within which we are completely unaware even of of our ignorance.
Ouch. If that doesn't knock you down a couple of notches, nothing will. Perhaps our arrogance is eclipsed only by our ignorance.
Unlike dark matter or its even stranger cousin, dark energy, dark knowledge is within our grasp if only we can break down our own memetic constructs to the point where the "holes" become apparent. The "holes" are the unthought thoughts, the missing concepts hidden in the gaps or falacies of our otherwise unexamined network of thoughts. Through this or other methods, exposed are the windows to all that lies truly "outside the box", to knowledge that cannot be arrived at only by building upon the knowledge within previous human experience. Dark Knowledge is connected to the trunk of all human experience only by some extensive branch that our ancient ancestors never explored, if it is connected at all.
To see through the window is not enough. To illuminate truly new knowledge, the window must be breached, the void traversed so that a bridge can be built to connect it back in to the trunk of cumulative human experience. This is precisely what Einstein did when he quantified his theory of relativity - he brought that knowledge out of the darkness by connecting his "vision through the window" to mathematical and physical concepts that were already understood. He had seen the answer long before he had the knowledge to substantiate his vision. He did not progress methodicaly from the known to the unknown. Instead, he grasped a truth well outside the box of previous human thought or experience and proceeded to build a bridge back to the known world.
It is thought provoking to note that civilizations may rise and fall, or simply vanish - and with them goes the threads connecting their knowledge to ours. In many cases, their "tools" (at least the ones we recognize as such) bear silent wisdom to this knowledge, effectively bridging the divide. But what of a society that develops an entirely different concept of tool, of technology? We could not hope to understand what we were looking at from an archeological perspective, and civilizations that may have been great, even transformational, could be easily dismissed as trivial if they did not leave behind comprehensible physical artifacts.