Dark data refers to the inactive or unused information stored by an...

https://wiki-planet.win/index.php/How_Does_Apache_Iceberg_Help_with_Unstructured_File_Metadata%3F

Dark data refers to the inactive or unused information stored by an organization, often making up 55% or more of all data. This hidden data increases storage and backup costs and creates serious risks like ransomware exposure since it’s rarely monitored