It's kind of a long-running debate whether to add one or two spaces after a period in a sentence. The practice of adding two spaces occurred when the typewriter was invented, when typists were forced to choose between one or two because of monospaced fonts (before that, typesetters could use differently-sized spaces). When proportional fonts came to rise and the computer invented, people started adding only one space. I've always used one space my entire life and never heard anything different, so I wanted to hear what you guys have to say.
I think it's a demographic thing where older people tend to use double spaces and younger people use single spaces, although I hear in some schools kids learn double spaces.