Songfacts®:

• Written and performed by the Barenaked Ladies, this is the title theme for the long-running CBS sitcom about two geeky physicists, played by Johnny Galecki and Jim Parsons, who invite the hot girl across the hall (Kaley Cuoco) to join their socially awkward circle of friends. The song is a rapid-fire history lesson about the creation of the universe that "all started with a Big Bang." A full-length version was released as a single in 2007.
• Before the show premiered in 2007, creators Chuck Lorre and Bill Prady were in the audience at a Barenaked Ladies concert when frontman Ed Robertson coincidentally improvised a freestyle rap about Simon Singh's book Big Bang: The Origin of the Universe. Shortly after, they contacted Robertson to write the theme song.
• "I actually wrote almost the entire thing in the shower one morning," Ed Robertson explained in a Songfacts interview. "I came up with the basic melody and most of the lines. I got out of the shower, made a really quick scratch demo just using GarageBand on my laptop, and I sent it to them and they loved it. Actually, they loved it so much they wanted to use that demo that I recorded like with a towel wrapped around me at the cottage. They loved the demo."
  
  "The only change they wanted was, I used to go [sings], 'We built a wall (we built the pyramids), Math, science, history, unraveling the mystery, that all started with a big bang. It all started with a big bang... BANG!' So, I tagged the last line, like sang it twice. And that was the only comment Chuck Lorre made. He said, 'Can we just not do the turnaround tag at the end, just go straight to 'It all started with a big bang... BANG'?' And I was like, 'Yeah, no problem.' That was it. That was their only comment on the song."
• So what exactly are autotrophs and why did they begin to drool? Robertson told us about the head-scratching lyric: "I was Googling 'early organisms' because I wanted to get basically from early single-celled organisms up to like way along the evolutionary chain in one line. So, I tried to get from autotrophs to mammals basically between 'the autotrophs began to drool.' So, the idea is just to mention the earliest kind of organism that sounded interesting and then get to sweating and drooling, defecating things, in one line."
• The guys were playing a gig at Universal Studios in Orlando, Florida when they added this to the set on a whim. The crowd loved it, and the song has been a staple of their live set ever since. It's included on the 2016 live album BNL Rocks Red Rocks, which was recorded at the Red Rocks Amphitheatre in Morrison, Colorado, during the band's 2015 Last Summer On Earth tour.
• Steven Page was with the band when they recorded this song, but left the group in 2009. In 2015, he sued his former band, claiming he hadn't seen any royalties from this song, which should be substantial considering how often the show aired, especially in reruns.
  
  The credits for the long version of the song list both Robertson and page as writers; the theme and end title list just Robertson. Page performed on the theme version though, and would be entitled to performance royalties for every airing unless there was an agreement of some kind. He claimed in the suit that the royalties should be divided equally among the five band members, but he saw nothing. In his lawsuit, it states, "One of the band's most successful songs, written by Page and Robertson, is entitled 'If I Had a Million Dollars,' and ends: 'If I had a million dollars, I'd be rich. Clearly, Robertson has taken this to heart and has kept not only his million dollars, but the millions of dollars belonging to Page and the other band members."
• The show's female lead Kaley Cuoco proved she knew all the words to this song when she sang it on The Tonight Show Starring Jimmy Fallon in a 2017 appearance.