'Big Bang Theory' Four Years: Cast Celebrates Success at PaleyFest 2013, Huge Ratings for CBS Sitcom, Season 6 Spoilers? [VIDEO]

The cast of CBS's hit sitcom "The Big Bang Theory" is celebrating its enormous success at PaleyFest 2013 in Los Angeles. The annual open-to-the-public cast and crew panel is running through Friday at the Saban Theatre. The cast of the show first showed up at the event back in 2009 when no one knew who they were. Now they are a part of another Chuck Lorre led production that has gained huge TV success.

Last week's "The Big Bang Theory" episode was the second most watched TV show overall with over 20 million viewers. The only show to beat the TV nerds was "NCIS." Created by Lorre and Bill Brady, the show has been a ratings hit since premiering back in 2007. The show is currently in its sixth season and will have its season finale May 16.

According to Nielsen ratings, "The Big Bang Theory" started with just over 8 million viewers an episode back in 2007 and has since grown to over 18 million viewers in its most recent season.

So what is the reason for the shows enormous success, besides the fact that it's funny? Apparently it's syndication. You may have noticed the show is playing all over the place. The show has been running in syndication on TBS since 2011 and now you almost can't go without running across it.

Although the cast and crew didn't reveal much at PaleyFest, actress Melissa Rauch (plays Bernadette) did say a letter from Howard's long lost father is found. "They (Howard and his scientist wife Bernadette) find the letter...Or someone finds the letter."

The shows main characters are Leonard Hofstadter,played by Johnny Galecki, and Sheldon Cooper, who is played Jim Parson. They are joined with friends and colleagues, engineer Howard Wolowitz played by Simon Helberg and astrophysicist Raj Koothrappali who is played by Kunal Nayyar. Kaley Cuoco plays Penny who lives across the hall from Leonard and Sheldon.

"The Big Bang Theory" airs Thursday nights at 8pm on CBS. So what do you think of "The Big Bang Theory?"