Automatically typing periods comes to the Mac from the iPhone and iPad world. To enable this feature:
• Go to the Apple menu and choose "System Preferences"
• Select the “Keyboard” preference panel, then choose the “Text” tab
• Locate the setting for “Add full stop with double-space” and toggle the checkbox to the "On" position
• Exit System Preferences
Now when you hit the space bar twice, a period and a space will be inserted automatically.