Thank computer sciences for the Internet! I was able to quickly discover that lacuna is an unfilled space, a missing portion, a gap, or a cavity. As long as I was on the Internet, I decided to look up the word's etymology and I discovered that, rather than being a newer word, it's been in use in English since the 17th century. It comes from Latin and is the diminutive form of the word from which we derive the word "lake." Basically, a lacuna is less than a lake--a hole without water. In the historical sense that the author was using it, it means that something should be there, but isn't.
I wonder about the lacunae that reside in our knowledge of one another. Even my own memories seem have a growing number of lacunae. I'm amazed at our minds' abilities to fill these empty spaces, sometimes with exaggerated versions of what actually happened, sometimes severely downplaying poor decisions that we have made. I suppose that it's in the filling of these missing portions of our knowledge that we truly see what sort of people we are. Do we tend to fill them with doom and depression, with possibility and hope, or with some combination thereof?
Ah, the paths that a single word can take me ...