Dark data is the information your organization collects but doesn’t use, often...

https://mill-wiki.win/index.php/What_Are_Transparent_File_Tables_and_How_Do_They_Work%3F

Dark data is the information your organization collects but doesn’t use, often making up 55% of all stored data. It increases costs through wasted storage and backup resources and raises breach risk since unused data is less likely to be secured