Dark data is the information companies collect but never use—up to 55% of...

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 information companies collect but never use—up to 55% of stored data can be dark. It increases storage and backup costs and raises ransomware and breach risks because unused data often lacks proper security controls