Big Bang Theory: What you never knew

The Big Bang Theory is celebrating its 10-year anniversary.

It's been 10 years since we first tuned in to watch Sheldon, Leonard, Penny and all the gang on The Big Bang Theory.

Premiering in 2007, the US television sitcom created by Chuck Lorre and Bill Brady initially received mixed reviews, and took a while to gather momentum in the ratings.

However, by its fourth season, The Big Bang Theory had become television's highest-rated comedy, just nudging out reigning champion Two and a Half Men.

So whether you were a fan from the start, or you've joined in along the way, here are some facts you probably never knew about the series.

There was a copycat show

During the third season of The Big Bang Theory, producers realised a television show called The Theorists in Belarus was ripping off their hit series.

They considered legal action, but it never eventuated because the stars of the copycat show all quit when they realised what had happened.

Kaley Cuoco wasn't in the pilot

Kaley Cuoco wasn't originally cast as the female lead in The Big Bang Theory.

She auditioned twice but missed out on the role to fellow actor Amanda Walsh.

However, after CBS gave the pilot episode the thumbs down, creator Chuck Lorre decided to mix things up and invited Cuoco to come back and test for the part again.

"Third time's the charm!" joked Cuoco to Vegas magazine about landing the role.

We still don't know Penny's surname

Most fans probably haven't even realised this before now — but they have never heard Penny's surname.

Unlike the other characters, hers has remained a mystery, which is reportedly at the request of Cuoco.

Co-creator Billy Prady has been quoted in the past as saying they'll reveal it eventually, but producer Steve Molaro has said the opposite — so who knows?

The pilot had a completely different theme song

The Barenaked Ladies' Big Bang theme song is iconic now, but it wasn't even the original choice for the show.

For the pilot episode, Thomas Doly's 1980s tune She Blinded Me With Science was used instead — but clearly it didn't stick, and was soon replaced with the current song.

Kaley Cuoco and Johnny Galecki dated for years

Kaley Cuoco is close to all of her Big Bang co-stars, but she's especially close to Johnny Galecki who plays Leonard on the show.

"We dated for almost two years," she revealed to Watch! magazine in 2010.

"It was such a huge part of my life and no one knew about it. It was a wonderful relationship but we never spoke a word about it and never went anywhere together.

"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."

The original theme song was very, very different.

Melissa Rauch was a huge fan of the show

When she joined the cast as Bernadette in season three, Rauch was already a huge fan of the show, but she tried to play it cool on set.

Variety reported earlier this year Rauch finalised her deal with Warner Bros. Television to stay with the series. The deal will reportedly pay $US500,000 per episode for 48 episodes.

Sheldon's shirts often represent his mood

Green Lantern fans are probably already aware of this, but for the rest of us: Sheldon is often wearing different-coloured shirts bearing the Lantern's logo but each shirt has a different emotion attached to it in the comic world.

For example — red is rage, blue is hope and indigo is passion.

A lot of the time, Sheldon's behaviour in a particular scene is mirrored by the colour of his shirt.