Dark data is the inactive or unused information that sits in your systems—often...

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

Dark data is the inactive or unused information that sits in your systems—often making up 60-80% of stored data. It increases storage and backup costs and raises ransomware and breach risks because it’s harder to monitor and protect