Dark data is the inactive or unused information stored in your systems that...

https://www.slideserve.com/aaron_ward92/why-does-dark-data-create-operational-drag-for-it-teams

Dark data is the inactive or unused information stored in your systems that often goes unnoticed. It can make up 60-80% of enterprise data, increasing storage and backup costs significantly. It also raises the risk of ransomware attacks and data breaches