Dark data is the inactive or unused information that piles up in storage—often...

https://tango-wiki.win/index.php/How_Do_I_Find_Unknown_Departmental_Shares_That_Nobody_Owns%3F

Dark data is the inactive or unused information that piles up in storage—often 60-80% of total data—yet still creates risks like ransomware exposure and contributes to $2.5 million in annual storage and backup costs