The English idiomatic phrase, “left in the dark,” refers to someone being unaware of a certain situation, or ignorant of certain factors. This is one of many similar phrases that use the metaphor of visual darkness for secrecy or lack of awareness. If someone is being left in the dark, he or she is not being informed about something when they should be.
English has many different phrases that use the same metaphor, and another common one is the use of the phrase “kept in the dark.” Although the meaning for this phrase is similar, when an English speaker says that someone is kept in the dark, this implies an deliberate effort to hide information or deceive the party, where being left in the dark is a condition of simply being passively uninformed. English speakers can use other phrases to cover other aspects of the same metaphor; for instance, if no one with inside information “sheds any light” on a subject, then everyone is “left in the dark” about it.
In general, the concept of shedding light on an issue relates to bringing out the truth in a certain matter. By the same token, English speakers also talk about “operating in the shadows” which again, refers to secrecy or discrete action. English speakers might also say someone acted “like a thief in the night” to indicate that someone did something with the intention of complete secrecy.
Other highly idiomatic phrases also relate to the use of light and darkness, or loss of sight as lack of knowledge. For example, when a group of people is “left in the dark” about something, there’s the idea of a whole group lacking knowledge and making ill-informed decisions. Another way to say this is to allude to the group of people “playing a game of blind man’s bluff.” In this classic game, one of the participants is blindfolded and cannot see. In the idiomatic phrase, the speaker is making a comparison of that one blindfolded person to the whole roomful of people, saying that all of them are “blind” to certain information, and thus operating “blindly.” These sorts of colorful expressions help speakers to talk more passionately and vividly about how a lack of knowledge can affect a group of people.