Lol, as soon as someone sees a slight change in a web page they jump to the conclusion that the site has been hacked. Why does this happen so often lol?
I dont think youtube is going to get hacked soon.. one of the top websites on the net and i am sure they got security of some sort
So as many here have said you have a simple rendering issue with your browser. YouTube didn't get hacked, you just didn't get all the data loaded properly. It looks like a CSS issue at first glance. Step one, clear your cache and try reloading the page. If that doesn't fix the issue, are you applying any custom CSS? If so, remove that and then reload again. There is nothing wrong with YouTube here.