Can someone explain the difference between .xlsb and .xlsx? I use .xlsb for all my files, but that was because it was suggested by my IT department so I can upload my large files to Sharepoint. It will dramatically reduce the size of my files...which is beautiful! But I would like to understand the difference because I am trying to get others in my office to follow me in using .xlsb. But the argument is, "then why didn't Microsoft make it their default?". Can you help me explain?