Dark data is the inactive or unused information organizations keep but don’t...

https://iris-wiki.win/index.php/Warm_Cutover_vs_Big_Bang_Migration:_Which_Is_Less_Risky_for_Your_Data_Migration_Plan%3F

Dark data is the inactive or unused information organizations keep but don’t analyze—often 55% of stored data. It increases storage and backup costs and raises ransomware and breach risks since attackers target forgotten files