Quick, clean, and to the point

Extend selection by one cell down

Windows shortcut 
Mac shortcut 

If multiple cells are selected, Excel will extend the entire bottom edge of the current selection.

Related videos

The videos below show how this shortcut can be used.
In this video, we show you how some of Excel's shortcuts for selecting cells.
In this lesson, we cover shortcuts you can use to extend selections.
In this video, we'll look at a way to fill in missing data using a formula created with the IF and ISBLANK functions.