Context: Game of Thrones, Dothraki expression. e.g. “Dragons are extinct. It is known.”
It has a nice ironic resonance. The use of the passive voice is quaintly overconfident, a sly tactical evasion of responsibility for whatever is claimed: it’s not me that knows, it is … other people, everyone. Ask anyone! It’s the same as “everyone knows that!” but more emphatic and clever: it is known, dammit, and that’s all there is to it!
I will now be using this ironically from time to time, as a snarky response to overconfident declarations — a little subtler and geekier than facepalm.