In one of my earlier blog posts, I said that when writing your plot, you need to make sure that the main character (MC) knows exactly as much as the reader. Which means that you can leave out a lot of information and facts as long as the MC doesn’t know them. Otherwise, not knowing for the reader would not feel justified. But that doesn’t mean that you, the writer, shouldn’t know.
The writer should always know everything. The writer should know why a particular character acts in a certain way; why the sun moves counterclockwise; and why the people on the other side of the river hates each other.
This also applies when the writer plans to keep such information a mystery; when no character in your story knows (or will ever know) about what’s going on. But that doesn’t mean the writer can be lazy and choose not to include theories because people will always speculate.
Basically, you should never consider a story finished if you can ask “why” at any point in your plot. This is how you flesh out a story.
Every action has a reaction, in the same way, there is no reaction without an action. Nothing should be implicit. Everything should be addressed. But that doesn’t mean everything should be explicit. Things can be subtle and mysterious without ever needing a clear answer, as long as the writer can answer these question.
Thus, there is no mystery for the writer. The writer is omnipotent; the writer is the universe; the writer is god!