What word would you use to describe something that is rarely known and understood?

The word "esoteric" is the most fitting choice to describe something that is rarely known and understood. It specifically refers to knowledge or information that is intended for, or likely to be understood by, a small number of people with specialized interests or expertise. This term often applies to subjects that are complex and not broadly familiar to the general population, making them exclusive and not widely discussed or recognized.

By contrast, the other options do not convey the same meaning. "Void" refers to an emptiness or absence, lacking the nuances of knowledge or understanding. "Tacit" describes something that is understood or implied without being stated outright; it does not necessarily pertain to the rarity of understanding. "Melancholy" relates to a deep, persistent sadness or gloom, which has no connection to the concept of being known or understood. Therefore, "esoteric" precisely captures the essence of something obscure or specialized in knowledge.