YouTube's left alignment is just...weird.

Who knows really why, but here are some theories:

Over time the default video size has change, due to either controls or the picture size, and nobody bothered to change the template which creates the container in that row.

Maybe an element requiring access or one that is dynamic in nature is intended to go there but the template was not designed to collapse that column when no content is present (assumption there is an empty col on the right, not just empty margin or gutter).

The video should be centered, not left justified, both for cosmetic purposes and if the video is expanded.

Sheer and utter stupidity in design - all elements function properly it simply could be better cosmetically.

I'm not in the mood to inspect You Tube to confirm any of this, so if you want definitive answers, use Chrome or any browser with decent inspection and right click into that blank area, examine the HTML and CSS and you'll see.

I assumed it was carryover from the days of 800x600 resolutions. I am fairly new to web design and development, but a lot of the older guys I work with align-left everything to accommodate screen resolution. Nothing wider than 800px - it drives me crazy.

As I said, I am new to the game, but that is my "out of my ass" guess.

Left alignment generally makes sense due to both screen resolution and users reducing browser window size pushing content to the left and up. Limiting to 800px is not as common these days due to higher resolution monitors and responsive theming for HD mobile devices. The W3C encourages a liquid or adaptive layout at minimum, even adjusting font sizes dynamically. But as we all know, layout is dictated by design so more often than not you'll see elements set with static width.

Video content is often static width and height with JS or Flash (being deprecated slowly, not fast enough to do so, my .02) so sometimes you end up with gutter space. The developers here chose not to display the video in a colorbox or modal popup (which I like) because there is a ton of adverts and crap they want you to play with and part with your money, aside from full screen mode.

Just some thoughts of mine, I'm sure others might disagree on some of what I said. Bottom line is I agree it could be done better on YT.

Simply Google's new brand guide that they have been slowly pushing over the last year or two. Left align is the new center according to Google designers.

Strictly brand/design/ui testing related.