Back in 2009, sitcom The Big Bang Theory was yet to be a hit. It didn't even make it into the Top-40-rated shows in the U.S.

Fast-forward to a week ago, when a first-run Big Bang Theory episode topped the list of shows viewed by 18- to 49-year-olds. This is impactful, as that market is the most coveted by advertisers.

The Big Bang Theory also boasts 20 million viewers, making it the second-most-watched show overall (after NCIS), according to a report published by Associated Press on Thursday, March 14.

The reason for the sea change? AP suggests it's all about syndication.

Millions of viewers who didn't catch The Big Bang when it first aired on CBS have been able to get hooked onto the series about a group of geeky guys thanks to syndication through local stations and cable network TBS. The episodes debuted in syndication in September 2011.

"You're kind of forced to watch the show now," Big Bang Theory star Simon Helberg said. "We're shoving it down people's throats, and then they're learning that they love that."

The Big Bang Theory was created by Chuck Lorre and Bill Prady (Dharma & Greg), and also stars Johnny Galecki, Jim Parsons, Simon Helberg and Kunal Nayyar.

"There's something impossible to fathom about viewers and numbers and things like that," Parsons said. "I can barely picture 20 people in one room. If you want to say 17 or 18 million, well you've lost me."

The members of the show's cast and crew celebrated their four years of success at PaleyFest, an annual open-to-the-public series of TV-series panel discussions. The fest runs through Friday at the Saban Theatre in Los Angeles.

The Big Bang Theory cast and producers declined to give any spoilers for future episodes, aside from Helberg's admission:

 "Well I think (Howard's) pants will probably be as skinny and probably a bit tighter."

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