Dark data is the information your company collects but never uses—often 60-80%...

https://wiki-site.win/index.php/How_Do_I_Stop_Dark_Data_From_Coming_Back_After_Cleanup%3F

Dark data is the information your company collects but never uses—often 60-80% of all stored data. This unused data takes up space, costs money to manage, and can hide security risks