Probably ice. Ice that falls into the entrance and then sublimates has a good chance of moving to perpetual darkness regions and to stay there. Factor a couple billion years and you probably will have a decent amount.
Now imagine there is some simple biology happening from the time the Moon had some atmosphere, in caves sealed billions of years ago.
I wasn't thinking about complex lifeforms, as there isn't enough energy in those dark caves to power anything complicated, but some rock-eaters could have thrived there.
It's remarkable the Moon has so many interesting stories waiting to be uncovered.
Now imagine there is some simple biology happening from the time the Moon had some atmosphere, in caves sealed billions of years ago.