"The Big Bang Theory" has continued to improve in the ratings race during its four years on the air.

Four years ago, the CBS sitcom didn't even crack the top 40 most-watched shows in the country. This week "The Big Bang Theory" was the top-rated show in its timeslot within the coveted 18 to 49 demographic the AP reports, drawing over 20 million viewers.

It's a number that actor Jim Parsons, who plays bookish physicist Sheldon Cooper on the show, can't wrap his head around.

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

One possible reason for the "Big Bang Theory" ratings growth over the last four years could be the show's move to syndication in September of 2011.

"You're kind of forced to watch the show now," actor Simon Helberg said Wednesday night at the PaleyFest arrivals line in Los Angeles. "We're shoving it down people's throats, and then they're learning that they love that."

With four years of "Big Bang Theory" behind him, Helberg, who plays engineer Howard Wolowitz on the show, offered his predictions for what season 20 of the show will look like.

""Well I think [Howard's] pants will probably be as skinny and probably a bit tighter," he said. "I think that Sheldon will probably still be sitting in his spot. It may be in a different apartment. I think he'll sit in that spot until he's in a home. Howard and Bernadette probably will...God, God help them if they have children. We know she hates them and we know that Howard, essentially, is a child. So I wish them luck."

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