谁知道【shéi zhīdào】

谁知道 as a reply means nobody can say – least of all the speaker. It is "who knows?", often with a shrug, and 谁知道呢 is the same with 呢 added.

It is a statement shaped like a question, so nobody answers it.

Who would have thought

In a story, 谁知道 opens the clause that overturns what was expected: "but then, to everyone's surprise". The first clause sets up the plan or the belief; 谁知道 brings the twist.

In writing, the shorter 谁知 does the same job.

Reading it as a real question

Neither use wants an answer. The surprise use needs something before it to overturn; on its own it is the "who knows" sense.

• × 谁知道他会来。 (meaning "to my surprise he came", with no setup) → 我以为他不会来，谁知道他来了。