Dark data is the inactive or unused information that piles up in your...

https://wiki-canyon.win/index.php/Why_Keeping_Dark_Data_on_Flash_Storage_Is_a_Bad_Idea

Dark data is the inactive or unused information that piles up in your storage—often making up 60-80% of total data. It increases costs by wasting storage and backup resources and raises breach risks since outdated files can be vulnerable to ransomware