Learn Turkish: Gök çok karanlık olacakmış. - Apparently the sky will be very dark.

It combines the verb olmak (to be / to become), the future suffix -acak, and the inferential (hearsay) suffix -mış. Together, they translate to "apparently it will be" or "I heard it will be."

Why do we need the verb olmak here instead of attaching suffixes directly to karanlık?

Karanlık (dark) is an adjective. To talk about what an adjective or noun "will be" in the future, you must use the helper verb olmak. You cannot attach the future tense suffix -acak directly to a descriptive word.

Does using the hearsay suffix -mış mean the speaker doubts that the sky will be dark?

Not necessarily! In Turkish, -mış is routinely used to pass along information you learned from an outside source, such as a weather forecast. It simply means "The forecast says..." or "I heard that..." without automatically implying doubt.

I usually see the word gökyüzü for "sky". Can I just use gök like in this sentence?

Yes, gök and gökyüzü (literally "face of the sky") both mean "sky" and are often used interchangeably in everyday speech. Gök is just the shorter, base word.