They're tiny by human standards. Zillions, if they exist, could dance on the point of a pin. But Weakly Interacting Massive Particles are called "massive" because they're at least 10 times--maybe as much as 1,000 times--heavier than protons and neutrons, the most familiar elementary particles in ordinary matter.
WIMPs are called "weakly interacting" because they seldom interact with anything. They don't even hook up with other wimps. Usually, they just glide through things without leaving a trace.
Not everyone welcomed the suggestion that something so alien and bizarre could be the main ingredient of dark matter, the invisible "glue" holding together the universe. Why, some astronomers lamented, are physicists so keen to dream up new particles when dark matter might be made of good old protons, neutrons and electrons?
"The idea that the majority of the mass of the universe was some exotic stuff we hadn't detected took a lot of getting used to," says Joel Primack, a UC-Santa Cruz physicist who helped introduce the WIMP theory in 1982.
WIMPs are disconcerting partly because they deal another blow to our species' self-image, observes Michael Turner, the University of Chicago cosmologist who coined the fanciful acronym. We've had to come to grips with the reality that we live on a small planet that circles a nondescript star in an out-of-the-way neighborhood of our galaxy--and now we're going to have to accept that we aren't even made of the same basic stuff as most of the cosmos. Turner calls this a "Copernican insult," alluding to the 16th-century astronomer who dethroned humans as the center of the universe by showing that the Earth orbits the sun.
As a less exotic alternative to WIMPs, some astronomers proposed in the late 1980s that a swarm of large, non-glowing bodies made from ordinary particles--hulking planets, burned-out stars, maybe even flotillas of black holes--lurked unnoticed in the halo of our galaxy. These hard-to-detect bodies could explain dark matter's gravitational pull, the argument went. Half in jest, astronomers called them machos, for MAssive Compact Halo Objects. The dueling particles were a headline writer's dream: WIMPs versus MACHOS in a battle royale to decide the ruler of the universe.
But MACHOS proved to be the feeble ones. In order for their gravity to hold our galaxy together, they'd have to be numerous--yet only a few have turned up in several exhaustive sky surveys that started in the mid-1990s. (Though "dark," machos are detectable because their gravity warps light coming from far-off stars.) What's more, the notion of dark matter composed of "normal" matter clashed with well-supported theories of how the elements formed shortly after the Big Bang, says Stanford physicist Blas Cabrera. By the late '90s, all but a few diehards had abandoned machos as an explanation of dark matter, Cabrera says.
With MACHOS down for the count, WIMPs were set to take their shot at the title.
