Premiere Elements 13/13.1 on what computer operating system?
The YouTube player is 16:9.
Are you using the Premiere Elements built in YouTube feature? If so, what specific YouTube preset are you using?
Or, are you exporting your Premiere Elements Timeline to a file and then importing that file to YouTube at the YouTube web site? In that case, what are your specific settings in the Premiere Elements export area?
Backing up for the moment...what did you or the project set as the project preset and what are the properties of your source files that are supposed to match the the project settings (project preset)? See Edit Menu/Project Settings/General and the readings there for Editing Mode, Timebase, Frame Size, and Pixel Aspect Ratio.
Please review and consider and supply more details.
Thanks for the excellent follow up information.
Based on what you posted, you have a 16:9 project that you are exporting to a 4:3 file which you are uploading to YouTube which has a 16:9 player. Result your black borders at YouTube.
Suggested solution: Export the Timeline content
with Presets = YouTube Widescreen HD
and then upload that file to YouTube at the YouTube web site.
I am not sure why you would want a MPEG2.mpg file for YouTube. But, you would want a 16:9 not a 4:3 preset.
with Presets = HD 720p30
I am not suggesting that you use this MPEG route, just giving an example of what you should have used to maintain the 16:9 project.
The project settings should match the properties of the files that you imported into the project. Do they? Based on the Edit Menu/Project Settings/General, either you or the project set project settings for a 1280 x 720 16:9 @ 29.97 progressive frames per second video.
Please consider and then let us know if the above worked for you. If any questions or need clarification, please ask.