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

https://garrettsbestblogs.theglensecret.com/how-do-i-check-if-dark-data-contains-pii-or-phi

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