This idiom means to be kept without information.
Notice that the pronoun is always inside the phrase: “to keep them in the dark” (not “to keep in the dark them“). 
A superstitious person believes in stories or traditions that have no explanation. Here are a few common superstitions you’ll hear in English:
something good)“So far I’ve done really well on each of my biology exams…knock on wood.”
“What a nice day for our hike…and not a cloud in the sky!”
“Don’t jinx us! Now it’ll rain for sure!”
“Adam’s won almost $200 in his card game, and he’s never even played before. Amazing!”
“Oh, I bet it’s just beginner’s luck. Just wait.”
“First my alarm didn’t go off this morning, then I spilled coffee on myself and now I’ve missed the bus home. Like they say, bad luck comes in threes…”
“Did you know that many American elevators go from the 12th floor to the 14th, and skip over the 13th? It’s thought that the 13th floor would be too unlucky!”
In your own words…
Tell us about some of the superstitions in your country. Are they very similar to these?
This little word has a lot different meanings:
1. key (noun): used to unlock a door or car
2. key (noun): the individual piece on a keyboard, used for typing on a computer
3. key (noun): the black and white pieces played on a piano
4. key (noun): group of musical notes – “The song is played in the key of E minor.”
5. key (noun): sth. that provides meaning or gives understanding – “The key to being a successful write is accepting that some of your work will be rejected.”
6. key (noun): a collection of symbols on a map or graph, telling you what the information means.
7. key (adjective): of great importance – “Martin Luther King Jr. was a key leader of the Civil Rights movement in the United States.”
8. under lock and key (idiom): in a protected place – “My mother keeps her jewelry under lock and key.”
When you learn a new word, check your dictionary to see if it can be used in different ways. It’s a very quick way to expand your vocabulary!
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