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

https://oscar-wiki.win/index.php/Tiering_vs_Archiving_-_What_Is_Safer_for_Inactive_Data%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