So far, scientists’ best efforts to detect dark matter have been thwarted. For now, they can only theorize about what it might be like, but it is most likely some kind (or perhaps several kinds) of subatomic particle.
The leading candidates are Weakly Interacting Massive Particles, or
s, any of hundreds of different subatomic particles which may have formed in the very early universe at very high temperatures. And there are others, with names like photinos, gravitinos, axions, and magnetic monopoles.
And there is the celebrated neutrino, long-sought by physicists, and the only candidate particle that has actually been detected – though it probably accounts for only a small fraction of the total dark matter, experts say.
Finally, some dark matter may be composed of
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| The cluster of galaxies EMSS 1358+6245 is about 4 billion light-years away in constellation Draco. When imaged with the Chandra X-ray Observatory, scientists determined the mass of dark matter there is four times larger than normal matter. Click to enlarge.
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| This first image of a dark matter object is a nearby red dwarf star. Six years ago, it gravitationally focused light from a blue background star in another galaxy in a so-called microlensing event. Since then, red dwarf has moved slightly in the sky and so is clearly separated in the new image.
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| A computer simulation shows the expected distribution of dark matter in the universe. Colored dots indicate galaxies, which appear to trace out the major features in the dark matter.
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|  | TODAY'S DISCUSSION | | |
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Where does that leave astronomers?
"As any scientist will tell you about any topic," says Trimble, "more work is needed," by both theorists and observers. For astronomers, she says, "that means doing a better job of monitoring, for instance, very high-energy gamma rays, very high energy cosmic rays" that could be produced when dark-matter particles collide with normal matter in the Milky Way. Detectors for such efforts are already operating underground in many locations around the world, including New Mexico and the Antarctic icecap.
And Chris Impey believes physicists’ efforts to detect dark matter particles could be decisive, even if they don’t find anything.
"I think within two to four years [researchers] will either have found the particles, or they will have constrained the favorite brand of particle to be too rare to account for all the dark matter. So we’ll have ruled it out. That means that progress is going to happen on the physics-detector front, regardless of what astronomers do."
Even so, Impey says, theorists will have their hands full with some puzzling observations of galaxies that raise doubts for the dark matter hypothesis. "Every simulation of the central parts of massive galaxies like the Milky Way predicts that the dark matter will form a highly concentrated central region." However, Impey says, "That’s not what’s observed. So there’s this emerging problem that won’t quite go away."
For all the uncertainty, however, astronomers don’t seem to mind the unsettled questions.
"The universe is there to make you humble," says Impey. "When I go observing I’m ready to be surprised. I don’t always figure we’re at the answers, or around the corner from the answers."
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