Learn Spanish (Latin America): El cielo está muy oscuro hoy.

We use está because we are talking about a temporary condition. The sky isn't always dark; it happens to be dark hoy (today) because of the weather. If you used es, you would be saying the sky is permanently a dark object!

Why is it muy instead of mucho?

Muy means very and is always used before adjectives (like oscuro) and adverbs. Mucho means a lot and is used with nouns or after verbs. Since oscuro is an adjective describing the sky, we need muy.

Would oscuro change if I was talking about a feminine word?

Yes! Like most Spanish adjectives, oscuro has to match the gender of the noun it describes. Since el cielo is masculine, it stays oscuro. If you were talking about la noche (the night), you would say: La noche está muy oscura.

How would I say that sky over there is very dark using the far-distance words from this lesson?

You would use aquel. Since cielo is masculine and singular, you would say: Aquel cielo está muy oscuro. You might use this if you were pointing at a distant storm cloud on the horizon!