In English, we have a phrase 'The elephant in the room'. It refers to a subject matter that is causing tension such as something awkward.
(Example situation: Two people have been known to do something together while drunk. The next day, the two people are in a group with their friends and everybody knows about it but nobody wants to be the one to mention it. In this case, whatever the two drunk people did together is "The elephant in the room")
Now, google translate tells me that I could say
L'éléphant dans la pièce
However, I believe the phrase is too idiomatic to translate literally and I presume that the literal translation would sound très bizarre to a native French speaker.
Any help that you can provide in helping me find a French phrase with similar meaning is greatly appreciated.