Do me a favor and take a look at your monitor screen. Not what's on it, but the monitor itself. More than likely - if you have a newer monitor - your screen is wider than it is tall.
An article from The Daily Dot has the audacious headline "You've been using your monitor wrong this whole time." The writer may have a point, but I'm not entirely convinced.
Did you know that computer screens have been following TV screen dimensions for decades? When computer monitors were first sold, they had a nearly square screen of 4:3 ratio. But what does this have to do with using your monitor differently?
Now that TVs have transitioned to widescreen, monitors have followed suit. This means that when you visit websites, there is a lot of white space on the edges of the website you're looking at.