Dark data is the inactive or unused information that sits in your storage—often...

https://romeo-wiki.win/index.php/Why_Does_Dark_Data_Create_Operational_Drag_for_IT_Teams%3F

Dark data is the inactive or unused information that sits in your storage—often making up 60-80% of files. It increases costs through wasted storage and backup expenses and raises breach risk since attackers target forgotten data