"Last night," recalls Pendulum's singer, songwriter, principal producer and "guitarist", Rob Swire, "there was a guy watching us who looked about 50. He was wearing a Pantera shirt, he's sort of that big" - Swire holds his hands six inches either side of his midriff - "and he was going nuts, havin' the time of his life. I thought, 'Fuckin' 'ell! If we're gonna smash it tonight, it's gotta be for that guy!'"

Pendulum have become 2008's biggest musical mystery. A band all but disowned by the scene that made them, they have sold 500,000 copies of their two albums. The hit of the summer festivals, they have received the thumbs-up from the dance and metal press, yet elsewhere, confusion reigns. They've effectively switched genres, a sin in certain musical circles on a par with changing your football team. Indie websites lambast them as poseurs, drum'n'bass diehards say they are sell-outs. An NME feature in July pondered, "Who's buying all those bloody records?"

The answer becomes clearer after a quick wander around the Guildhall in Portsmouth, during an early date of their sold-out UK tour. There is a visceral excitement to Pendulum's stage show - a blend of rock theatrics, state-of-the-art lighting and a punishingly loud, crystal-clear sound - that helps it transcend genre boundaries. The band have rejected the traditional route of playing for purists in favour of a technology-savvy approach that prioritises reaching out to everyone, all the time.

Beyond age - most of the crowd are in their teens - there is little to unite the disparate tribes. There are girls wearing pink feathers and Lycra workout gear; shirtless lads slam-dancing and covered in sweat and lager; metal fans in rock band T-shirts; rave casualties in white gloves with glowsticks; and emo kids in black hoodies subsuming themselves to the Pendulum assault.

"I think it's the fact that we've taken great care trying to perfect the sound," Swire says, trying to analyse his band's appeal. "Also, it's a bit of a cliche, but the other half of it is the whole sensory overload thing. At these shows there's 14-year-old kids, and I can't imagine they've been to that many gigs before. And if this is their first gig ..." Then Pendulum have just made every other band's job exponentially more difficult.

The combination of drum'n'bass arrangements, metal dynamics, hulking riffs carrying monstrous melodies and Swire's dark, ambiguously portentous lyrics is not just winning, it may be unique. Plenty of bands have tried to meld rock and dance, but barely a handful have managed to create music that blends different forms so seamlessly. The Prodigy, Reprazent and Justice have trodden similar paths, but they didn't consummate their marriage of musics with an album whose title nods to a rock touchstone (Pendulum's second album, In Silico, is an acknowledgment of Nirvana's In Utero) and which emphasises its makers' attempts to fuse human input with machine music (the pseudo-Latin of "in silico" was coined to refer to biological experiments simulated on computers). Pendulum, it seems, are more than just genre-manglers on a lucky streak: they are connecting because their music sounds, and feels, like the future. As Granite, the sci-fi themed, set-closing In Silico single, puts it: "This is a new way."

Formed by Swire, Gareth McGrillen and Paul Harding in Perth, Australia, Pendulum became unlikely drum'n'bass pioneers after their first single, Vault, was embraced by the scene's major figures in 2004. "I've loved the whole technology thing since I was little," recalls Swire, who does most of the group's interviews despite a shyness at considerable odds with his dominant stage presence. "Drum'n'bass combined that with the attitude you could find in punk: finally, there was an electronic genre that had all that without being completely abrasive."

From their isolated location - "Perth was always, like, six months out of date," Swire says - the trio felt their dislocation keenly, reliant on the occasional gig by one of drum'n'bass's leading figures or the internet to keep on top of the rapidly moving scene's creative curve. Technology played a vital, viral role.

"I was watching a live video stream from the End in London of Ed Rush playing one of his sets," says Swire. "He started playing his first record, and it was Vault. This is one of the guys who you idolise in drum'n'bass, and he just starts off his set with your record. I was sitting there in Perth, watching him playing it, going, 'Jesus Christ!' We didn't know anyone had it, although I knew Paul had emailed it to a couple of DJs. Obviously, they'd been sending it to each other."

When drum'n'bass artist Dan "Fresh" Stein offered them the chance to live in his London flat rent-free for eight months while he went to the US, Swire and Harding moved to England. Eventually, McGrillen followed, and the group were offered an album deal by Breakbeat Kaos, Stein's label. Touring to promote their debut album, 2005's Hold Your Colour, would change the band beyond all recognition; Harding became a standalone Pendulum DJ (enabling Pendulum to play different cities on the same night), while techno-head Swire, in particular, was encouraged in his increasingly ambitious attempts to graft the human and computerised elements of their music.

"We didn't know Paul Kodish from a bar of soap," recalls Swire, "but we met him and he said, 'When I listen to Hold Your Colour, to me it sounds like it's a band playing it.' He used to play drums in Apollo 440, and suggested trying to play tracks from the album live. The more we thought about it, the more exciting it was, and then everything else fell into place. It was, 'OK, we've got a live drummer, so we'll have a guitarist to play the bits that are played by guitar; I'll play keyboards, not just tweaking things on a computer. And the vocals? We'll try and do them live as well.'"

Trying to find the equipment that offered the optimum in both sound and swagger did not come cheap. Among Swire's flashier purchases is the £2,500-worth of custom-built "guitar" he uses on stage. Called a Z-Tar (pronounced "Zee-tar"), it's actually a MIDI controller that can be played like a guitar, but which sends wireless signals to his keyboards. A highly flexible tool, its principal attraction was that it looked a lot more rock'n'roll than the alternatives.

"I've gotta be honest - we did actually buy a Keytar," he admits of the hopelessly uncool round-the-neck synths fashionable in the 1980s. "It probably woulda worked, but you just can't use one on stage - the 80s ruined that, unfortunately. But I saw a video of [jazz guitarist] Stanley Jordan playing these massively complex organ parts on two Z-tars, and I was thinking, 'Shit, if he can do that, I sure as hell can play a fuckin' Pendulum riff on one!'"

It was the new all-live Pendulum - Swire, Kodish, McGrillen on bass, guitarist Perry ap Gwynedd and MC Ben Mount - that In Silico was written for. "There was a time that we were making music just for the drum'n'bass community," says Swire. "While we were writing a track we'd be thinking how it would go down at Fabric on a Friday and the End on a Saturday. But by the time we'd come round to writing In Silico, and we already had the live show together, we had to completely stop minding what the drum'n'bass scene felt."

The evolution was organic, but didn't appear that way to outsiders. Severing ties with the scene that had made them, though risky, felt like the only option. "It definitely caused more than a few sleepless nights," Swire allows. "But if we'd just kept writing drum'n'bass, I would've felt like that was selling out. Even though some people in the drum'n'bass scene think otherwise, this direction feels the least compromised."

By the end of the set, the Portsmouth crowd are spent. The wall of lights at the back of the stage is transformed into a matrix screen displaying a three-minute countdown, and by the time the clock reaches the last 20 seconds, recharged fans shout each descending number, ready to plunge back into the room-sized mosh pit.

Granite is saved for last. "Granite was originally about invasion and things changing," Swire concludes. "I didn't really see it as being about us until it was finished, and then I looked at the lyrics and I thought, 'Fuck, that could be interpreted as us talking about ourselves.' I hope other bands start getting inspired by the idea of doing electronic stuff 100% live. Just the thought of electronic people not relying on a backing track, not pressing 'Play' on shit and just letting a sequencer play the whole thing while they tweak and filter - that's what excites me about the future of bands."

• Pendulum play Brixton Academy, London, tonight. Box office: 08444 77 2000. Then touring.