A riddle: What rules the pop music world and costs less than a buck? If you haven’t listened to Top 40 since 18-year-old Avril Lavigne was teething, you’re probably scratching your head. But anyone else knows that the answer is rap newcomer 50 Cent and his uber-smash, “In Da Club.”

During one week in March, the song reached 170.2 million radio listeners, according to the industry trade magazine Billboard. “It’s the most listened-to song in chart history,” said Silvio Pietroluongo, who oversees Billboard’s Hot 100 ranking of popular songs.  The tune’s success helped 50 Cent’s album “Get Rich or Die Tryin’ ” sell 872,000 copies in its first four days of release — a record for a new artist. 

All this has turned a relatively simple song about having fun at a nightspot into a cultural phenomenon. “It’s a record that has reached damn near everywhere,” said Billboard’s Minal Patel, who monitors the airplay of rap and rhythm-and-blues songs.

A large part of 50 Cent’s popularity comes from his persona and pedigree. As a former drug dealer who once survived being shot nine times, he comes with an intriguing back story. He is signed to Shady Records, owned by mainstream rap superstar (and recent Academy Award winner) Eminem. And “In Da Club” is produced by Dr. Dre, who, like a modern-day Berry Gordy, has an uncanny ability to make records that appeal almost equally to black and white audiences.

“Club,” for example, has a throbbing low-end bass sound that’s perfectly in line with hard-core hip-hop aesthetics. But at the same time, the beat has such a steady thump that even the most rhythmically impaired person can keep time to it.

“It’s the kind of beat that doesn’t just get related to the clubs,” said Patel. “It translates to radio.”

“The beat is it,” said V-103 radio disc jockey Frank Ski. “It just comes off so hard.”

Already, at least three other artists have recorded spinoffs of “In Da Club,” which borrow its instrumental track: Destiny’s Child frontwoman Beyonce Knowles, R&B superstar Mary J. Blige and rapper Bubba Sparxxx.

Knowles’ remake — which was pulled from the airwaves after 50 Cent’s record company issued a “cease and desist” order — finds the glamorous diva boogieing through some hip joint draped in diamonds and sipping Bailey’s Irish Cream. Blige, who received permission to redo the track, takes a more romantic approach, crooning about a cute guy she met — where else — in the club. And Sparxxx, a white rapper from Athens whose music is often dubbed “hick-hop,” calls his take “In Da Mud.”

Of course, it’s nothing new for big hits to beget remixes, parodies and other alternative versions. (In 1999, TLC’s no-account-man-dissing “No Scrubs” provoked up-and-coming rap group Sporty Thievz to produce the answer record “No Pigeons.”) But what is significant — indeed unprecedented — about the 50 Cent remakes is that they’re done by established artists instead of burgeoning acts looking to ride a trend. For Knowles, Blige and Sparxxx, all of whom have new albums coming out, using the “Club” beat is a way for them to prove they’re still relevant.

But the reach of “In Da Club” doesn’t end there. On a recent Saturday night, it was the most requested tune at Atlanta gay nightclub the Heretic, which usually plays up-tempo dance music. Katie Couric bopped to it one morning on NBC’s “Today” show. And it showed up several times in a recent MTV documentary about the then-impending war in Iraq.

Upon arriving in Kuwait City, MTV news correspondent Gideon Yago marveled that “Club” was the first song he heard on the radio. “If there’s any culture shock I’m going through,” Yago said, summing up the experience, “it’s pretty much an absence of culture shock.”

“In Da Club” — with its expansive, continent-hopping, genre-leaping, age-defying reach — is the type of tune that pop music champions live for. Its popularity seems to sum up everything good about pop music: how it can bring people together in spite of race, gender, sexual orientation, culture and politics.

The things it celebrates — drinking, dancing and having fun — are so basic that it’s nearly impossible to be opposed to them. But, at a time of national division and global crisis, you have to wonder whether this is a good thing or really naive.