Dark data is the inactive or unused information stored in your systems that...

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

Dark data is the inactive or unused information stored in your systems that often goes unnoticed. It can make up 60-80% of enterprise data, increasing storage and backup costs significantly. It also raises the risk of ransomware attacks and data breaches