Dark data is the inactive or unused information stored across your systems,...

https://extra-wiki.win/index.php/What_is_a_Global_Namespace_and_Does_It_Help_with_Dark_Data%3F

Dark data is the inactive or unused information stored across your systems, often making up 55% of all data. It ties up storage and backup resources, leading to unnecessary costs and raises breach risks since attackers target overlooked files