'Bang!' Cuoco secretly dated co-star for 2 years

"The Big Bang Theory's" Kaley Cuoco sure can keep a secret!

The California-bred actress, 24, opens up about her undercover relationship with costar Johnny Galecki in a new interview with CBS Watch Magazine.

Photos: Undercover lovers

"We dated for almost two years. It was such a huge part of my life and no one knew about it," Cuoco reveals. "This is the first time I've ever talked about it, ever. It was a wonderful relationship, but we never spoke a word about it and never went anywhere together."

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Photos: Shocking celebrity splits

The sitcom star says she and Galecki, 35, decided to keep things under wraps because they were "so protective of ourselves and the show, and we didn't want anything to ruin that." But according to Cuoco, the secrecy eventually a toll on their relationship, causing them to split last winter.

Photos: Stars who fell in love on set

"We couldn't do anything. It wasn't as fun as we wanted it to be," she says. "Everyone was always asking, and we deny-deny-denied. And I'm like, 'Why am I denying this person that I love?'"

Photos: Better after breakup?

Despite the breakup, Cuoco says things haven't been awkward on the set of their hit CBS show.

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"We knew we weren't destined to be together. We accepted it and said, 'Look, if we ever break up, we will be professional,'" she recalls. "I'm so lucky it was a mutual ending and that we don't hate each other. We've seen that happen on other shows."

Photos: Stars who found hot rebound romances

Things aren't looking so hopeful for their respective characters either, who also split at the end of last season.

"I do think they're meant to be, but it's gonna be a while from now," she says of Penny and Leonard. "And I like that they're broken up. Penny needs some other dates. As does Kaley!"