Double spacing was used from the typewriter days when text was fixed spaced. Since the full stop (period) took the same space as a capital M it wasn’t necessarily clear as to the end of a line. So too with a comma etc. Obviously now that typewriters are in the museum and most of us use word processors, all text is proportionally spaced, making a double space after a full stop obsolete.
One way of changing a document to single spacing is to do a search and replace; searching for a double space and replacing with a single.
Esteban: Nonsense! I’ve been typing since 1988 and was taught to double-space after my periods. I had never heard otherwise until I installed Microsoft Office 2007 on my PC and MS Word started marking my double-spaces as grammatically incorrect.