This normally occurs due to the aspect ratio of the program or movie you are watching. Depending on the aspect ratio the program or movie was originally recorded, this may be normal. However, if your TV has a WIDE mode function, it can be used to adjust how the video displays on the TV screen; many widescreen TVs have a WIDE button on the original remote control.
If watching programming from a connected cable or satellite set-top box, you may need to configure the aspect ratio output in the set-top box menu. Aspect ratio (aka TV Type) settings are typically found under Settings, Options, Preferences, TV Option, or another similarly-named location in a set-top box menu.