Dark data is the inactive or unused information your business keeps, often...

https://wiki-neon.win/index.php/Dark_Data_in_Object_Storage_%E2%80%93_Why_Do_Buckets_Get_Out_of_Control%3F

Dark data is the inactive or unused information your business keeps, often making up 60-80% of stored files. It raises breach and ransomware risks since unused data is less monitored. It also leads to storage and backup waste, costing millions annually