The Impersonal Subject

Hello teachers,
The context:
[color=blue]Quickly he ran down the hill. It was very dark here, and he could not see very well.
The question:
Why could he not see very well?
He could not see very well because it was very dark.
My questions:
I think the ‘it’ in the answer is the ‘dummy subject’ because I have a form of ‘be’ and an adjective. On the other side, the answer expresses an opinion about the place, doesn’t it?
Am I right in my assumptions?

Thanks in advance.

I agree that ‘it’ in your sentence in terms of syntax has the function of a grammatical subject without referring to anything in particular and is sometimes called ‘dummy’ or ‘prop’.

Hello Alan,
Thank you for your reply.