Rhetorical questions with 谁, 什么…: 谁不知道啊？
【四63】强调 / emphasis
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
A rhetorical question built on a question word expects no answer: it asserts the opposite of its form. The polarity rule is simple: a negative verb means "everybody" (谁不知道啊？: everyone knows), a positive verb means "nobody" (谁知道呢？: nobody knows).
什么 has three common uses: 有什么 + noun = "no such thing" (我有什么办法？: I can't do anything about it); verb + 什么 = "no need to" (你急什么？: no need to panic); 什么时候 + verb + 过 = "never" (我什么时候说过？: I never said that).
The question closes with 啊 or 呢, never 吗. It is spoken and expressive, from teasing to irritation: natural between friends, but curt with a superior. This point adds to the 哪儿 and 怎么会……呢 rhetorical questions you have seen, and differs from 难道……吗, which keeps 吗 and expresses surprise.
Examples
这么有名的歌手，谁不知道啊？
Zhème yǒumíng de gēshǒu, shéi bù zhīdào a?Such a famous singer, who doesn't know him?
明天会不会下雨，谁知道呢？
Míngtiān huì bu huì xià yǔ, shéi zhīdào ne?Will it rain tomorrow? Who knows?
他总是迟到，谁还相信他的话啊？
Tā zǒngshì chídào, shéi hái xiāngxìn tā de huà a?He is always late, so who still believes what he says?
你别问我了，我有什么办法？
Nǐ bié wèn wǒ le, wǒ yǒu shénme bànfǎ?Stop asking me, what can I possibly do?
离飞机起飞还有三个小时，你急什么？
Lí fēijī qǐfēi hái yǒu sān ge xiǎoshí, nǐ jí shénme?The plane doesn't take off for another three hours, what's the rush?
我什么时候说过我要去？
Wǒ shénme shíhou shuō guo wǒ yào qù?When did I ever say I would go?
Common mistakes
✗ 谁不知道吗？ → ✓ 谁不知道啊？ (never 吗 after a question word; 啊 or 呢 closes the question).
✗ 谁知道啊？ (meaning that everybody knows) → ✓ 谁不知道啊？ (a positive verb says "nobody", a negative verb says "everybody").
✗ 老师，您急什么？ → ✓ 老师，您别着急 (the rhetorical question is curt, kept for friends and family).