Elephant In The Room Meaning

(idiomatic) A problem or difficult issue that is very obvious, but is ignored for the convenience or comfort of those involved.

Example: 2008, Nicholas Rufford, "Motormouth: The car of the future, at £1,300," Sunday Times (UK), 6 Jan.,
  There is an elephant in the room that nearly every politician and green campaigner is ignoring. It’s called population growth.