If it’s not anitmatter, what is “dark matter?” Good question.

We’re clear that dark matter is not antimatter, right?  It can’t be, or it would constantly annihilate any “regular” matter that wandered by, and we’d be able to detect the effects of those mini-catastrophes.  Instead it just sits there, unseen, not interacting with light and barely ever touching the normal kinds of stuff we know.  Hard to figure a guy out with just the impression his ass leaves on a couch cushion.  More on that later.  Antimatter we understand, partly because its existence was predicted mathematically by the legendary Paul Dirac in 1928, before it was ever observed “in the wild,” an insight for which he won the Nobel Prize in Physics.   Once the numbers came up, we actively went looking for it, and physicist Carl D. Anderson acquired the confirmation 4 years after, separately also earning his field’s top honor.

We came at dark matter differently, as instead of predicting it, the stuff took us by surprise.  Our conception of dark matter began largely as an ad hoc placeholder for a funny observation of the outermost edges of galaxies.  According to traditional orbital mechanics, stars way out there should move much slower than the ones closer in, as there isn’t as much mass acting on them.  While that’s true, those speeds didn’t trail off nearly as sharply as calculated, a puzzling discovery made by Dutch astronomer Jan Oort, the namesake of the “Oort Cloud” of comets that envelop our solar system, in 1932 (see image below).  Fritz Zwicky made a more precise observation a year later when analyzing the rotation curve for the Coma cluster of galaxies, and determined there should be more than 100 times more mass in the farthest regions than was visible.  Zwicky decided there had to be unseen (unseeable?) matter out there gravitationally pulling those stars along.  That’s a pretty big assumption.  Could something else be at play?

Rotationcurve_3from asterisk.apod.com

In 1983, Israeli physicist Mordehai Milgrom wondered if maybe the calculated mass discrepancies exist because we don’t understand how Newton’s law of universal gravitation operates at great distances, so that it’s the equation that’s wrong, and there really is no need for “invisible matter.”  It wouldn’t be the first time Newton came under fire from advancing physics.  Does relativity ring a bell?   Since Milgrom proposed his Modified Newtonian Dynamics, though, dark matter has scored a significant win with the phenomenon of gravitational lensing.  If dark matter is massive enough to change the speeds of distant stars, than it should be able to bend light, as well.  The smearing of the galaxies in the below image is due to the bending of light as it passes through the Abell 1689 galaxy cluster, as observed with the Hubble Telescope.  Thing is, without dark matter,  there just isn’t enough mass to account for all that distortion.  There must be something we don’t see.  Observations of the cosmic microwave background radiation seem to further necessitate the presence of the mysterious material.

Grav. lensfrom wikipedia

So while we can now be pretty sure that dark matter is a real thing, we’re still unable to get much of a handle on it.  Most of it seems to be where we first noticed its effects, in enormous “halos” surrounding galaxies.  Recent research suggests however that it could be strung throughout all of space, and earthbound experiments are counting on picking out a few particles to further examine its nature.  The Cryogenic Dark Matter Search (CDMS) of the University of California at Berkeley made waves last month when they announced three possible dark matter signatures detected in 2007 and 2008.  The CDMS utilizes silicon and germanium crystals cooled to near absolute zero, and operates under the idea that on the rare occasion that a bit of dark matter does interact with regular matter, the displacement it causes in the crystal can be measured.  The whole apparatus is buried deep underground to minimize false positive detections.  Similar experiments using water have been used to detect neutrinos, as does the new COUPP-60 dark matter experiment.

WHAT DOES THIS MEAN?

Figuring out what dark matter is proves to be a lot more difficult than showing that it is.  In addition to terrestrial detectors, the Alpha Magnetic Spectrometer on the International Space Station got some attention in early April when project scientists revealed they may have evidence that dark matter is mostly made of theoretical particles called neutralinos.  How did they make that determination?  With antimatter!  The new measurements show more positrons in cosmic rays than anticipated, which some speculate are produced from the collision of dark matter in deep space.  I guess the two things have hidden connections after all.

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