Dark data refers to the 55% of enterprise data that is stored but never used....

https://blast-wiki.win/index.php/How_Do_I_Decide_What_to_Keep_for_Compliance_vs._What_to_Remove%3F

Dark data refers to the 55% of enterprise data that is stored but never used. This inactive data increases storage and backup costs and raises ransomware exposure since attackers often target overlooked files