Elephant in the room
= (idiom) a major or controversial problem that is not discussed. [idiomeanings.com]
= An important and obvious topic, which everyone present is aware of, but which isn't discussed, as such discussion is considered to be uncomfortable. [Phrases.org.uk]
= a serious problem that everyone is aware of but no one wants to talk about. [Macmillan]
Assunto delicado que ninguém quer falar a respeito, fantasma que está rondando.