What to call the "things that nobody talks about"?

I am writing an article about “things that nobody talks about.” I feel sure that there should be a single word which can replace the phrase things that nobody talks about? It’s rather clumsy as it stands, in my opinion.

They could be taboos, or merely secrets. People probably refer to them with euphemisms. In any case, they are unspeakable.

Perhaps not exactly what you're looking for, but 'elephant in the room' is an idiom that you can use when talking about a single instance of something that is an obvious, unavoidable truth that people still insist on not talking about.

The phrase basically stems from the idea that having an elephant in your living room is a large and obvious problem - but it's easier not to talk about or confront it because it's ultimately a problem that can't be solved without a great degree of difficulty.