Matt and Kim - Lo-fi popsters are walking on sunshine

Of all the many reasons to like or love a band – the tunes, the lyrics, the beats, the dancing, the good looks – there are few whose USP is sheer joy. Yet it's impossible to describe the Brooklyn duo Matt and Kim in terms other than the happiness they exude. It translates into their perma-grinning personas and uplifting, anthemic tunes; into their riotous, everyone-go-crazy gigs; their zany let's-get-naked music videos; and the acclaim as "the smiling-est band in the world."

Indeed, the videos for their shouty, catchy-as-hell songs – already watched more than six million times online, excluding the global Bacardi TV-ad campaign using their new single "Daylight" – have prompted a startling divergence of opinion.

"Their music just makes you feel so good and alive," one fan posted on YouTube, "and makes you want to get up and dance!" But there are the haters. "They think they're so cute," wrote another. "They make me sick." But the ever-engaging and ever-obliging Matt and Kim are just having fun. It's what rock'n'roll music used to be all about. And boy, do they have fun. They would make great children's entertainers. They even recorded their new album, Grand, in Matt Johnson's childhood bedroom.

This is also a band who play together, eat together and sleep together. They even finish each other's sentences without interruption or contradiction. And, until very recently, they'd shared a single mobile phone for seven years.

Johnson, the male half of the duo, anticipates the next question: "I know – we live together, we work together, do everything together, so we should hate each other," he says. "We should totally have killed each other by now. But we really get along."

"It's crazy," agrees Kim Schifino. "It's not even like we come home from tour and say, 'I'll see you in a week.' We come home and we're like: 'So you wanna watch a movie now? You wanna eat?' We really spend all our time together."

After spending a day in their company at the Pitchfork Festival, it's hard not to wonder whether their symbiotic togetherness is some sort of artworld prank by a smart indie-world Gilbert and Georgina. So who wears the trousers in this perfect relationship? "Matt has only two jobs," sums up Schifino firmly. "Taking out the garbage and answering the phone. And I do everything else."

Their minimalist musical palette – two small keyboards and three drums – fails to convey the exuberance of their sound. There's not a lot to it in conventional musical terms (how could there be?) but their feel-good vibe, enhanced by constant onstage banter, and spur-of-the-moment ideas like Johnson injecting "The Final Countdown" into a song in Chicago (watch it on YouTube), are contagious.

"We've broken music down very simply to two elements – melody and beat," explains Johnson. "Something you can sing along to and something you can bob your head to. You can get into it without thinking about it."

So that's the secret of their music. Now what's the secret of their happiness? Perhaps it's because they were already friends and lovers by the time they formed a band. "I think it's actually easier to find people who are proficient instrumentalists to put a band together than to find people who can spend 25 hours a day together," reasons Johnson.

They met at a Brook lyn art school seven years ago where he, a teenager fresh from rural Vermont, was studying illustration, and she, a mature student from Providence, Rhode Island, took film-making. So did their eyes meet across the campus grounds? "Her eyes met my butt," grins Johnson, while Schifino heartily endorses this: "I gave him my number three times before he called."

Johnson bought himself some vintage 1980s keyboards and taught himself to play, using one like a bass, and the other to play the equivalent of guitar power-chords. Schifino got herself a small drumkit. They started out playing DIY shows in and around their home base of Brooklyn ("Grand" is the name of the street where they live) but came to wider attention with their eye-catching videos, which they conceived themselves. First came "Yea Yeah", in which they perform while being pelted with food from their fridge. By the end of the single-shot take, they have managed to change their clothes – in mid-song.

Their next video for "Lessons Learned" has just been nominated for MTV's Breakthrough Video award. Also shot in one fluid take, it sees them jump out of a van and walk smilingly through Times Square while divesting themselves of all – yes, all – their clothes.

Startled tourists and office workers stare in disbelief as the duo dash down the street discarding garments until, with perfect (but entirely unplanned) timing, two cops intervene to arrest them and struggle on the pavement with the now-naked duo.

Schifino took some convincing to go ahead with the public strip-tease: "I started working out after that video! The video was Matt's idea but he had to persuade me for a really long time. I just didn't want to get naked and I didn't want to get arrested. I'm a really strait-laced square."

'Grand' and new single "Daylight" are out now on Fader Label/Nettwerk. Matt and Kim tour the UK in November. www.mattandkimmusic.com