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

https://quebeck-wiki.win/index.php/What_is_a_Global_Metadatabase_and_Why_Would_I_Want_One%3F

Dark data is all the information your company stores but never uses—like old emails, logs, or files gathering dust. It can be up to 60-80% of your data, quietly eating up storage space and costing you money—sometimes $2.5 million a year