Dark data is the inactive or unused information that builds up over time, often...

https://mag-wiki.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 that builds up over time, often making up 60-80% of stored data. It can increase storage and backup costs and raise the risk of ransomware attacks or data breaches