Aug. 3, 2007 — The mysterious dark matter that's been called on to make sense of the ways galaxies twirl through space may not exist, if an alternative theory is right.

The surprising way galaxies rotate — as if they are much larger and heavier than they appear to be — has long implied to astronomers and astrophysicists that there is more matter out there holding things together than we see.

That unseen and unseeable matter has fallen under the catch-all term "dark matter." These days, the most likely candidate for what makes up dark matter is some sort of weakly interacting particle that we've so far failed to detect.

But there is another radically different possibility: What if gravity itself doesn't work quite the way we think? Maybe at the outer edges of galaxies where the gravitational acceleration — the g — of a galaxy is extremely small, gravity tugs just a tad bit more.

If so, that miniscule difference could be enough to cancel the need for dark matter altogether. That's what the Modified Newtonian Dynamics (MOND) theory suggests, astronomer Stacy McGaugh explains in an article in the Aug. 3 issue of the journal Science.

The Hubble telescope spies a new class of planets.

"It's definitely not getting enough attention," said McGaugh of MOND. One reason is that the tiny difference in g which MOND requires is hard to detect. "Even in the inner solar system, the effect is negligible."

To put it into perspective, the g McGaugh is talking about is 1/100 billionth of 1 g. The gravitational acceleration we feel at the surface of the Earth is 1 g, and the Earth feels about 1/10,000th of a g from the sun.

Some scientists think the MOND g has already been detected by NASA monitoring the Pioneer 10 and 11 spacecrafts, which are now 40 to 60 times further from the sun than Earth.

"They show an extra acceleration directed towards the sun at about the same order of magnitude," said McGaugh.

But he's skeptical, since even a change in the solar wind or even a tiny fuel leak could create the same effect. Also, it's not at all clear that the Pioneers are far enough away to be under the influence of the altered MOND g.

Nor is MOND the only alternative.

"Another possibility is that the dark matter particle is somewhat different than in the standard theories," said astrophysicist James Bullock of the University of California at Irvine. "Of course, I could be wrong and Stacy could be right. This is exactly why we need to keep obtaining new data and making ever more accurate predictions."

New telescopes now being built or planned could clear up the matter soon, Bullocks said.

"This is an exciting area of research that will prove enlightening — one way or the other — over the next 10 years or so," he added.

---

Get more information on dark matter:

A NASA primer on dark matter.

Stacy McGaugh's homepage.

A scientific review of the latest dark matter evidence.

Explore the science of dark matter with the University of California, Berkeley.