The real reason Kaley Cuoco and Johnny Galecki split

If you're a big star in Hollywood, it's hard to keep your private life under wraps. However, Big Bang Theory co-stars Kaley Cuoco and Johnny Galecki managed to pull off one of the biggest secret relationships in Tinsel Town.

The stars, who play Penny and Leonard on the hit CBS comedy show, revealed to the world in 2010 that they had been dating for around two years, but had split on friendly terms. People were shocked – how could they keep it so secret for so long? "It was such a huge part of my life and no one knew about it," Cuoco said at the time, while Galecki gushed about his former flame in 2013, "She's not an ex, she's a part of my life."

While Leonard and Penny are still in love and even walked down the aisle on The Big Bang Theory, their real-life relationship fizzled out off-screen. Let's a look at the real reasons the secretive lovebirds ended their illicit romance.

They were forced to go into hiding

After confessing to dating her co-star in an interview with CBS Watch! magazine back in 2010, Cuoco also revealed she never went anywhere with Galecki during their two-year romance, for fear of being photographed together.

"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. We were so protective of ourselves and the show and didn't want anything to ruin that. But that also made it sad, too," Cuoco confessed.

"That's not the kind of relationship I want – I don't want to be hiding. We couldn't do anything. It wasn't as fun as we wanted it to be," she explained.

Penny and Leonard forever

Galecki opened up about the romance in 2013, and called his former flame "a dear friend". However, he went on to reveal how he and Cuoco were worried their off-screen romance would distract from the on-again/off-again relationship their characters Leonard and Penny had on the hugely successful show The Big Bang Theory.

"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 told CBS Watch! magazine.

Despite their off-screen relationship ending, their beloved characters Penny and Leonard ended up getting married in the first episode of season nine, which aired in September 2015, and then wed again in season 10 which aired in 2016.

They had to lie to their friends

The cast of The Big Bang Theory are all good friend, as they're often seen sharing sweet snaps on social media of them goofing around on set. But despite their tight-knit friendship with their co-stars including Jim Parsons, Kunal Nayyar and Simon Helberg, Cuoco and Galecki still chose to lie to their friends about their blossoming romance. Eventually, it affected their relationship.

"Everyone was always asking, and we deny-deny-denied. And I'm like, "Why am I denying this person that I love?" It took a little bit of a toll on me and I think it did for him, too… We knew how to keep it deep inside and never bring it on that set, ever," Cuoco explained.

They even worked during the aftermath of their break-up, telling nobody on set they had ended their relationship. "We broke up before the holidays and worked that whole time. We dodged a huge bullet. Dodged a war," Cuoco added.

Being former child stars haunted their love

Cuoco started acting at the tender age of three when she appeared in the Denzel Washington sci-fi thriller Virtuosity (1995), while Galecki caught the acting bug aged just six years old, and got his big break at 14 when he was cast as Chevy Chase's son Rusty in National Lampoon's Christmas Vacation (1989).

Having dealt with the pressures of Hollywood longer than most, the couple were desperate to keep their romance away from the prying eyes of the paparazzi and celebrity gossip magazines. However, it was a decision that ultimately ended their two-year-long romance.

"When you're dealing with two professionals who have been working since they were both four – and who kept their lives extremely private – we didn't want anything other than that," Cuoco said of their relationship.

A doomed romance

Despite their admiration and respect for one another after their break-up, Cuoco shockingly revealed the couple had actually agreed how they would deal with ending their relationship when they first began dating.

"We knew we weren't destined to be together. We accepted it and said, 'Look, if we ever break up, we will be professional,'" the actress recalled. "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."

Better friends than lovers?

Since their break-up, the couple have continued to be close friends, even meeting each other's partners and goofing around at awards shows. Were they just always destined to be better as friends than as boyfriend and girlfriend?

"We're dear friends, still," Galecki told CBS Watch! magazine in 2013. "She's not an ex, she's a part of my life."

Rumors swirled that the pair were dating again after Cuoco split from her husband Ryan Sweeting in 2015, but the on-screen lovers took to social media to shut down the untrue reports. "No scandals/home wreckers/pregnancies here, y'all. Just profound friendship," Galecki captioned a shot of him on Instagram with his former girlfriend, while Cuoco joked, "Leonard and Penny will just have to suffice!"