The expression "the elephant in the room" (or "the elephant in the living room") is a metaphorical idiom in English for an important or enormous topic, question, or controversial issue that is obvious or that everyone knows about but no one mentions or wants to discuss because it makes at least some of them uncomfortable and is personally, socially, or politically embarrassing, controversial, inflammatory, or dangerous. The metaphorical elephant represents an obvious problem or difficult situation that people do not want to talk about. It is based on the idea and thought that something as conspicuous as an elephant can appear to be overlooked in codified social interactions and that the sociology and psychology of repression also operates on the macro scale.

Source: Wikipedia