According to Wikipedia:
“Elephant in the room” is an English metaphorical 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.
The Oxford English Dictionary gives the first recorded use of the phrase, as a simile, as The New York Times on June 20, 1959: “Financing schools has become a problem about equal to having an elephant in the living room. It’s so big you just can’t ignore it.”