Dark data is the inactive or unused information that piles up in your...

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

Dark data is the inactive or unused information that piles up in your storage—up to 60-80% of it. This data not only wastes money on storage and backups but also increases your risk of ransomware attacks and data breaches