Dark data is the inactive or unused information stored across your systems,...

https://iris-wiki.win/index.php/What_is_Garbage_In_Garbage_Out_(GIGO)_in_AI_Data_Prep%3F

Dark data is the inactive or unused information stored across your systems, often making up 60-80% of total data. It ties up costly storage and backup resources, and if left unmanaged, increases ransomware and breach risks