“Elephant in the room” is an English idiom for an obvious truth that is being ignored or goes unaddressed. The idiomatic expression also applies to an obvious problem or risk no one wants to discuss.
It is based on the idea that an elephant in a room would be impossible to overlook. Thus, people in the room who pretend the elephant is not there have made a choice. They are choosing to concern themselves with tangential or small and irrelevant issues rather than deal with a larger one.
The term refers to a question, problem, solution, or controversial issue that is obvious, but which is ignored by a group of people, generally because it causes embarrassment or is taboo. The idiom can imply a value judgment that the issue ought to be discussed openly, or it can simply be an acknowledgment that the issue is there and not going to go away by itself.
The phrase “800 lb gorilla in the room” is a similar idiomatic expression. However, it refers to a large, unstoppable individual or organization that can exert its will as it desires, even if people do their best to ignore it.