Most people design their slides for a the 4:3 aspect ratio of older TV screens and computer monitors (I still think it is actually better than 16:9).

Increasingly conference rooms are using large 16:9 LCD screens that are much brighter than the traditional on screen projectors. And most of these 16:9 screens are set to stretch 4:3 input signals. As a result your slides will look bloated.

Grab that monitor remote (you cannot control this from your computer) and set the aspect ratio to 4:3 before you start presenting. The tech person present usually will say “What, you want those black bars?”. You can answer affirmative. Your slides will look much better, and if you use a black slide background, no one will even notice the black vertical bars.

P.S. Ancient post that touches on slide backgrounds.