Hello everyone, it is me again, who asks for help with cases

I came across this sentence :

Я опять дома
I understand that дома is in genitive form, but I can't understand why it is ? My book doesn't say anything about it. ( Or maybe I just don't understand something )

Anyone could shed light on why it is in genitive form here, and what is the " rule " for this, if there is one ?

thank you in advance