The yawning void is everything in the universe that is outside of one's current conceptual schemes.
For example, consider people in cultures who lived 1000 years ago on a diet of mostly grains and legumes (like rice and beans). Little did they know, their diet was necessary for them to live because neither grains nor legumes alone have all eight essential amino acids, but the combination of the two does. From the eight essential ones, your body is able to manufacture the remaining twelve.
Could they have guessed that?
Could they have considered the possibility and searched for it with the aim of ruling out all possible alternatives?
Could they have systematically checked to find out what was important about eating grains and legumes at the same meal when living on a vegetarian diet? Out of the huge Search Space of possible things to check that were relevant to their lives, could they have even considered checking that it might have something to do with amino acids?
Once you see how some knowledge you have is in other people's Yawning Void, you can see how nearly everything, including things of interest to you, is in your Yawning Void.
And then you can start thinking about fruitful ways to search the Yawning Void.
It can't be searched for known goodies
You can't ask intelligent multiple-choice questions about things in the Yawning Void. For example, "What color is it?" is not an appropriate question to ask about protons.
All you need is one person who grew to hate beans or rice, or became allergic to one of them. That person would have eventually gotten ill and died. If multiple people did it, a dietary problem may very well have been seen (so could a "spiritual" problem). Witness Gregor Mendel, who didn't know the specifics of what he was testing for (genes), but was still able to make certain statistical probabilities as to it's properties. Barring a prodigious chemist, they wouldn't have discovered what amino acids are, but they would have eventually been able to understand a great deal of why amino acids are important (and possibly what systems individual amino acids are most important for). It's possible certain beans and or rice were purposefully added to the diet once it was realized going off a hunter-gatherer diet was potentially dangerous to one's health (mixing and matching until a workable solution was found).
I don't see how this opposes the idea of this page. The idea is that no one could have thought a priori of the possibility that amino acids are necessary for life as one candidate hypothesis and systematically searched to ruled out all possible alternative hypotheses. The reality here was in the natives' yawning void. They would need surprising events, like someone becoming allergic to beans and then become malnourished--events that you can't think of in a systematic manner in terms of concepts you already have. They could only get to the real explanation via a long and convoluted and surprise-filled path. That's because the real explanation was in their yawning void. And that's why I think this illustrates the concept. --Ben Kovitz