Elephant in the room
Etymology
The image of an animal too enormous to overlook yet somehow unmentioned appears in 19th-century writing, and the exact phrase “elephant in the room” spread through 20th-century American English. Its power lies in the absurd contrast between the thing’s size and the group’s pretense of not seeing it.
noun phrase · /EL-uh-funt in thuh room/
An obvious, weighty problem that everyone present notices but no one is willing to mention.
What it names
The shared, deliberate silence around something too big and awkward to acknowledge.
Example
The whole meeting danced around the layoffs, the elephant in the room no one would name.
Related idea
The elephant in the room is an obvious truth left unspoken; pluralistic ignorance is subtler, where each person wrongly thinks they alone see the problem, and an open secret is known to all but kept from outsiders.