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

https://wiki-cafe.win/index.php/What_Is_a_Global_Namespace_and_Does_It_Help_with_Dark_Data%3F

Dark data is the inactive or unused information stored across your systems, often making up 55% of all data. It poses a real risk by increasing exposure to ransomware and breaches and drives up storage and backup costs, sometimes wasting millions annually