Question

why
had Europeans called Africa A Dark Continent

Answer 1

Africa got the name ' dark continent ' in the 19th century by the Europeans as they themselves weren't aware much of their own country. It's land, riches, resources etc.... 
So that's why they were called so.
Thanks

Answer 2

because it used to have so much unexplored/unmapped land.
so Europeans called Africa A Dark Continent

short and sweet

hope this help you