"Nobody had known it."
Translation:Ingen hade vetat det.
And correctly so. Ingen hade känt det can only mean Nobody had felt it.
känner can mean know in sentences like Känner du henne? = 'Do you know her?'
But for knowing facts, like in this sentence, we say veta.
No, visst means "sure", or is a negation. It's not a form of veta.
In this specific case, there is no other form of vetat you could substitute it for.
Edit, an hour later:
(The original post said visst, not visste. Please don't edit your posts so it looks like I'm an idiot for replying to something you didn't write.)
As I wrote, there is no other form of vetat that works here. That goes for visste, too, which means "knew". It's the same as in English: you can't say "nobody had knew it."
So "Ingen hade känt det" does not work, but why does "Ingen hade känt till det" work then?