"Elephant in the room" as I've heard it used is a particularly poignant metaphor.

Picture a roomful of visitors calling on someone who has recently lost a loved one to cruel illness, and everyone says "How are you?" but doesn't want the answer, and there's lots of small talk, all conspicuously avoiding the most obvious and important thing.

No one wants to mention it, but it's very clear, and everybody has to lean over awkwardly to avoid it; it's holding the conversation around an elephant in the middle of the room.