A productive starting-point for an overview of unknowns is recognizing that unknowns are socially constructed. Mine is among the earliest such statements (Smithson, 1985) and you can see my definition of "ignorance" in Smithson (1989, Ch 1), which embeds it in social relations.
To get an idea of how difficult it is to define any ignorance-related concept without recourse to socio-cultural norms, see my blog post on the topic of delusions.
Secrecy and lies are obvious ways that unknowns get socially constructed, and in another blog post I dip my toe into that large topic.
Anyone who has raised children has encountered the need to control their child's curiosity. Curiosity may or may not have killed cats, but it certainly can prove problematic, if not fatal, for humans. The social and political control of curiosity is another instance of socially constructed (or perhaps, reinforced) unknowns. Among the concerns that drive disputes about whether humans should inquire into various matters is the "dual use dilemma", whereby a piece of research or technological development can be used for great good or great evil. The link in the preceding sentence leads to my blog post on this topic; I also have a chapter in a forthcoming book on the dual use dilemma, edited by Michael Selgelid and Seumas Miller.
When I first started publishing material on ignorance and uncertainty in the 1980's, the "knowledge society" and "knowledge explosion" were buzz-phrases. It occurred to me that in some important respects we also were moving towards an "ignorance society" because of a corresponding "knowledge explosion" that drove everyone towards hyper-specialization and away from any kind of "big picture". A couple of decades later, Sheldon Ungar (2008) wrote a neat paper about this, and I commented on it a few times in my blog, including this post.
References:
Smithson, M.
(1985). Toward a social theory of ignorance.
Journal for the Theory of Social Behavior,
15: 149-170.
Smithson, M. (1989). Ignorance and Uncertainty:
Emerging Paradigms. Cognitive
Science Series.
Ungar, S. (2008). Ignorance as an under-identified social problem.
The British Journal of Sociology, 59, 301-326.