Dark data is the inactive or unused information sitting on your storage, often...

https://wiki-coast.win/index.php/Why_Do_We_Have_So_Many_Copies_After_Backups_and_Replication%3F

Dark data is the inactive or unused information sitting on your storage, often making up 60-80% of your files. It matters because this data can expose your business to ransomware attacks and data breaches while driving up storage and backup costs