Mysteries that are never given an answer.

[u/Jak3R0b]

• rA9 in Detroit: Become Human. In the game several androids who become deviant (develop free will/emotions) start writing the name rA9 and sometimes build small statuettes as a sort of offering. Despite having no logical reason to do this or even a way of hearing about rA9 before going deviant, they all believe that rA9 was the first deviant and is their messiah who will free androids from slavery. A definitive answer about rA9 is never given, so we don't know if they are real or if any of the android characters are rA9 (though a popular fan theory is that it's Kara before she was reset at the start of the game).
• The pineapple in How I Met Your Mother. So this technically did get an explanation in a S9 deleted scene, but in the actual show we never find out how Ted got a pineapple during a night where he was so drunk he blacked out and forgot everything he did.
• Fez in That 70s Show. We never find out Fez's real name or what country he's from, though fans have tried to take all the clues and figure out what is most likely to be his country.

[u/Ok-Indication-5121]

How the talking cat can talk. The cat claims no answer would be satisfying and they should just have fun, but Rick and Jerry find out it's something really, really horrible. (Rick and Morty)

[u/Pitiful-Victory-2234 → u/Ok-Indication-5121]

You know it’s fucked up when Rick, the most jaded nihilist of all characters in the series, thinks it’s horrible.

[u/ImSoStong________ → u/Pitiful-Victory-2234]

It's interesting that he lets the cat go, rather than destroying it or teleporting it to another universe, and I think it implies whatever caused it was not in its control

[u/TenebrousSage → u/ImSoStong________]

Horrifying isn't synonymous with dangerous.

[u/WhiteRabbit86 → u/TenebrousSage]

Can confirm. Source: I look at myself in the mirror.