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

https://wiki-nest.win/index.php/Tiering_vs_Archiving_%E2%80%93_What_Is_Safer_for_Inactive_Data%3F

Dark data is the inactive or unused information that piles up in your storage—often making up 55% of all enterprise data. This unused data increases costs through wasted storage and backups, and it raises your risk of breach or ransomware exposure