Dark data is the inactive or unused information that piles up over time—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 piles up over time—often making up 55% of storage. It increases your risk of ransomware attacks and bloats backup costs, sometimes costing businesses over $2.5 million annually