Tartaria refers to the mostly uninhabited eastern part of Russia.
Go look at an old map, it was the eastern part of imperial Russia.
And the dark ages refers to a time hundreds of years earlier.
The fall of the Roman Empire onwards, it's called the dark ages because Europe regressed into warlordism and monarchism.
I think it lasted like 600 years. But the rest of the world was still doing their thing, the Arabs and Chinese made a lot of scientific discoveries during the European dark ages.