What is a word that means "something that is commonly known, but not commonly talked about"?

Solution 1:

Open secret — M-W

noun a supposedly secret but generally known matter

"It's an open secret that he's been cheating on his wife."

Open secret — Wikipedia

An open secret is a concept or idea that is "officially" secret or restricted in knowledge but is actually widely known; or it refers to something that is widely known to be true but which none of the people most intimately concerned is willing to categorically acknowledge in public.

Solution 2:

It's not single-word, but there's a good fit in common knowledge.

something that is known to many people but often not made known officially

Cambridge

Solution 3:

As you say "It could not be of English origin", the adjective taboo comes to mind.

late 18th century: from Tongan tabu ‘set apart, forbidden’; introduced into English by Captain Cook.

Definition: "not acceptable to talk about or do" (Mirriam Webster)

examples:

Filipina helper Liz (not her real name) battled bed bugs in June 2012 in Tai Wong Street in Wan Chai. She says the pests covered the 500 sq ft hostel, which was home to 10 helpers. "They were on the walls, on my towels and on the carpet." Travellers also spread the problem. Bed bugs are a bigger issue in hotels than the industry will admit. "People don't talk about it because it's taboo, but it's definitely happening" http://www.scmp.com/lifestyle/health/article/1402401/bed-bug-infestations-are-rise-and-pests-are-hard-kill

Get kids talking about the loo, say NGOs, and it will help break down a major barrier to education in the developing world: cultural taboos about sanitation. https://www.theguardian.com/global-development-professionals-network/2015/nov/19/reading-writing-and-sanitation-how-kids-are-key-to-ending-toilet-taboos

Solution 4:

unspoken truth (thanks be to @hatchet)

Also, as a variation on the term unspoken rule, we could coin the phrase unspoken culture.

Finally, Carl Jung's collective unconscious seems apt here.

Wikipedia

According to Jung, the human collective unconscious is populated by instincts and by archetypes: universal symbols such as the Great Mother, the Wise Old Man, the Shadow, the Tower, Water, the Tree of Life, and many more.

Jung considered the collective unconscious to underpin and surround the unconscious mind, distinguishing it from the personal unconscious of Freudian psychoanalysis. He argued that the collective unconscious had profound influence on the lives of individuals, who lived out its symbols and clothed them in meaning through their experiences. The psychotherapeutic practice of analytical psychology revolves around examining the patient's relationship to the collective unconscious.

Solution 5:

Tacit \Tac"it\, a. [L. tacitus, p. p. of tacere to be silent, to pass over in silence; akin to Goth. [thorn]ahan to be silent, Icel. [thorn]egja, OHG. dag[=e]n: cf. F. tacite. Cf. {Reticent}.] Done or made in silence; implied, but not expressed; silent; as, tacit consent is consent by silence, or by not interposing an objection. -- {Tac"it*ly}, adv. [1913 Webster]

       The tacit and secret theft of abusing our brother in
       civil contracts.                         --Jer. Taylor.
 [1913 Webster]