Galecki and Cuoco had a secret relationship for two years ending in December 2009, running a bit ahead of the curve of their characters Leonard and Penny (via AOL). The secretive nature of their romance was based on a protectiveness of the show and its characters, not out of any shame — but it may have contributed to their breakup. "It was such a huge part of my life, and no one knew about it," Cuoco said in a now-deleted CBS Watch interview. "It was a wonderful relationship, but we never spoke a word about it and never went anywhere together." (via Today)

Added Galecki, "I just don't like to speak about it. And not because I'm trying to be enigmatic; I just worry that it will conflict with people's acceptance of Leonard and Penny ... I get the curiosity, but I don't want to distract from the story." He made it incredibly clear, however, that Cuoco is, and always will be important to him. "We're dear friends, still. She's not an ex, she's a part of my life." 

Cuoco clearly agreed, telling Haute Living, ""We weren't weird, which is what was weird; it was a mutual breakup, and you can rarely say that." She added, "We got over it really fast, and we're closer than ever now. It could have gone either way, and I was really proud of us."