"The elephant in the room" is a popular phrase that has traditionally referred to an obvious major issue or problem that nobody is talking about because it is taboo or would be too embarrassing to acknowledge.
According to Wikipedia, "In 1814, Ivan Andreevich Krylov (1769-1844), poet and fabulist, wrote a fable entitled 'The Inquisitive Man' which tells of a man who goes to a museum and notices all sorts of tiny things, but fails to notice an elephant. The phrase became proverbial. Fyodor Dostoevsky in his novel 'Demons' wrote, 'Belinsky was just like Krylov's Inquisitive Man, who didn't notice the elephant in the museum....'
Lately, I've seen it used to mean simply the biggest problem or issue, not one that is unnoticed or ignored.
I'm fascinated by how language changes, and this may be an example of a phrase in transition. There's a logic to this change, but some usages and patterns appear mysteriously. For instance, why do so many people now start a sentence with the word so, especially when answering a question? It seems to be replacing "well" as a way of buying a moment while you formulate your statement.
PS: A headline that may have been the result of watching too many political debates appeared today, about a new computer mouse with "unpresidented" ergonomics." In the same article, Julian Horsey (who may not have been responsible for the headline) calls it "very unique." That's another word in transition--unique means one of a kind, so nothing can be very unique or a little unique, but increasingly the word is being used to mean unusual.