Dark data is the inactive or unused information sitting on your storage, often...

https://smart-wiki.win/index.php/How_Do_I_Query_Object_Store_Metadata_in_Databricks%3F

Dark data is the inactive or unused information sitting on your storage, often making up 60-80% of your files. It matters because this data can expose your business to ransomware attacks and data breaches while driving up storage and backup costs