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

https://smart-wiki.win/index.php/Dark_Data_in_Object_Storage_-_Why_Do_Buckets_Get_Out_of_Control%3F

Dark data is the inactive or unused information that piles up in your storage—often making up 60-80% of total data. It increases costs by wasting storage and backup resources and raises breach risks since outdated files can be vulnerable to ransomware