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

https://mike-wiki.win/index.php/What_Is_Intelligent_Tiering_and_When_Should_I_Use_It%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