It is idiomatic. "Il fait" for lighting or lack thereof. "Il est" is incorrect, and you would be misunderstood for saying a person or thing is dark.
So "C'est sombre" would mean something specific that is dark, rather than weather?
Yes. "C'est" would be used with something specific other than the weather or ambiance, and it can mean gloomy, somber, or shadowy (figuratively).
C'est un homme au visage sombre. = He's a man with a dark countenance (gloomy face).
C'est une sombre histoire de trahison et de meurtre. = It is a grim story of betrayal and murder.
Here are some links to Larousse and Word Reference for additional uses of "sombre"
My response of "It's dark out" was not accepted. Since the sentence starts with "il fait", I believe that means it's referring to weather, rather than a person or thing. So the meaning "It is dark" should be acceptable because it's generic and might refer to weather or some other object, but I think "It's dark out" should also be accepted.
"Il fait sombre/noir" does refer to the lack of light rather than an object as you surmised. It's dark! However, I'm not really sure if "out" should be accepted or not since words that do mean "out" and "outside" (dehors/à l'extérieur) are not a part of this sentence...and they could have grammatically fit at the end of this sentence.
The "il fait + ..." is idiomatic in French.
Il fait beau = it's a nice day.
Il fait du soleil = it's sunny.
Il fait jour = it's daytime/it's daylight.
Il fait mauvais = it's bad weather.
Thanks for your response! It seems to me that if "Il fait du soleil" means "It's sunny" and it's never truly sunny inside, then similarly "il fait sombre" is likely to mean "it's dark outside". But this wouldn't be the first time DuoLingo failed to agree with me on a nuanced difference. Anyway, your response was helpful. Thanks again.
You're welcome! Personally, though, I agree that it is very natural to say "it is dark out."
Edit: On further reflection "out" should be omitted since it can be dark indoors as well. Mea cupla.
If the statement can refer to a dark basement, then I would say adding "out" would be incorrect.
You are correct, it can refer to the ambiance indoors. Thus "out" would not be correct in all contexts.
The impersonal "il fait" is used to express weather in French. Here it doesn't mean "he makes/does", but rather the "weather is/it is."
Whenever you see "il fait" followed by beau, mauvaise, sombre, froid, chaud... you can be sure it is a reference to the weather.
Just like English verbs take on multiple roles, so too do French ones!
la lumière tombe/il commence à faire noir = it's getting/becoming dark.
I wrote exactly the words "It is dark!" and the system said I was wrong. This happened twice. What gives?
why doesn't "it is getting dark" or "it gets dark" work as well? "il fait" has a sense of becoming too, isn't it? or you should just use "Il devient" instead?
Why "It gets dark" is not accepted?
My dictionary gives 'cloudy' and 'overcast' as alternatives for 'sombre'. So why doesn't Duo accept 'It is cloudy'?
Le temps est nuageux is how you'd say it is cloudy. Il fait sombre does not really imply clouds or cloudy weather, it simply means it's dark. It could be dark because of the clouds, but it isn't a given.
"Il fait" can mean "it is" or "the weather is". In this case, "the weather is" is a clearer meaning and should be accepted.
It is dark (outside).
[Il fait is an idiomatic phrase used strictly with reference to the weather]
The weather is dark.