Dark data is the information your company collects but never uses—like old...

https://wiki-nest.win/index.php/Does_Dark_Data_Affect_Backup_and_Disaster_Recovery_Costs%3F

Dark data is the information your company collects but never uses—like old files or unused backups. It matters because up to 55% of stored data is dark, leading to wasted storage costs and hidden security risks