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All of a sudden, the sky was dark.

All of a sudden, the sky in the dark.

Which is correct?

Why?

Thank you
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This is OK
All of a sudden, the sky was dark.
and the only one idiomatic.
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All of a sudden, the sky in the dark.

There's no verb here. So, logic aside -- there are things that can be in the dark and things that can be in the light, but the sky is not one of them -- that's not a sentence.

The sun dropped behind the building, and all of the sudden the playground was in the dark.