The elephant in the room
Solution 1:
An 'elephant in the room' has been used in a variety of senses for 'something obvious and incongruous' since the first recorded example in 1935. (OED)
However it is only since as recently as 1984 that it has been around in the sense for which it is nowadays most often used, namely 'for a significant problem or controversial issue which is obviously present but ignored or avoided as a subject for discussion, usually because it is more comfortable to do so'. (OED)
The full OED entry is worth reading.