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

https://wiki-wire.win/index.php/How_Do_I_Check_If_Dark_Data_Contains_PII_or_PHI%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