Dark data is the unused information stored in your systems—often 55% or more of...

https://wiki-spirit.win/index.php/How_Do_I_Keep_Sensitive_Files_Out_of_an_AI_Pipeline%3F

Dark data is the unused information stored in your systems—often 55% or more of all enterprise data. It can increase storage and backup costs by millions annually and raises ransomware and breach risks since it’s rarely monitored