The default file extension or the default format that is used in Microsoft Excel is (.xlsx). You could open the Microsoft Excel file with (.xlsx) format even in Google Docs, Apple Numbers, and OpenOffice.
If you are wishing to learn Microsoft Excel, and want to get certified in it, then check out the Excel classes from Intellipaat, as this offers you with industry grade course on Microsoft Excel, and helps you in mastering it, to optimize your skills and aid you in getting good opportunities. Have a look at the following video and get your fundamentals correct on Microsoft Excel.