Dark data is unused or forgotten information that builds up in your...

https://alpha-wiki.win/index.php/How_Do_I_Quantify_What_Dark_Data_Is_Costing_Us_Each_Year%3F

Dark data is unused or forgotten information that builds up in your systems—often 55% of all stored data. It increases storage and backup costs and raises breach risks like ransomware exposure since outdated data isn’t properly managed