For many years, writers were taught to include two spaces after periods and certain other punctuation marks, such as colons. There was good reason for this at the time: Typewriters produced a typeface that made it aesthetically unpleasant to have only one space separating sentences.
However, those days are long gone. The fonts computers produce have made the two-space method unnecessary. Today, every major style manual (including the Chicago Manual of Style and the AP Stylebook) makes it clear that one space is proper.
Does it matter if you still use two? Actually, yes. Anytime we see two spaces in an article that was written in recent years, we immediately think “amateur hour.” And that’s the last thing you want your reader to think about you.