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

https://spark-wiki.win/index.php/How_Does_Apache_Iceberg_Help_with_Unstructured_File_Metadata%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 It increases costs by wasting storage and backup resources and raises breach risks since outdated files can be vulnerable to ransomware