The Search for Dark Matter

A $2-billion experiment on the International Space Station has released the first data from its unprecedented census of the charged subatomic particles whizzing by Earth. Although the results, presented April 3 at a seminar at CERN in Geneva, largely confirm previous observations, researchers hope they will lead to discovering the identity of dark matter, an invisible form of matter that outweighs normal matter in the universe by more than 5 to 1.

The Alpha Magnetic Spectrometer is the latest and most ambitious attempt to uncover the identity of dark matter by looking for cosmic rays, which are charged subatomic particles cruising through space. Theoretical physicists have proposed that dark matter could be made up of exotic particles that can slam into and annihilate each other, creating detectable cosmic rays such as electrons and their antimatter partners, positrons.

This first batch of AMS results, published April 3 in Physical Review Letters, encompasses about 25 billion particles detected over the course of a year and a half, including 6.8 million measurements of the electrons and positrons that could come from dark matter. AMS improved the precision of earlier data, detected particles at higher energies than previous instruments and found that the particles arrive in equal amounts from all directions.

But none of the new data give clues to the positrons’ source, said Katherine Freese, a theoretical astrophysicist at the University of Michigan in Ann Arbor. The trajectories of these charged particles can change as they move through magnetic fields, she said, making it difficult to determine where the particles began their journey. Her bet is that rapidly spinning stars called pulsars produce positrons and fling them across the galaxy using extremely strong magnetic fields. “It will take a while to sort this out,” she said.

In the mid-1990s, physicists got a first peek at cosmic rays that could have resulted from dark matter annihilation. The High Energy Antimatter Telescope, a cosmic ray detector attached to a high-altitude balloon, found an unexpectedly high number of positrons, a result that seemed to jibe with the idea that dark matter annihilation creates these charged particles. In the last five years two space-based detectors, PAMELA, for the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics, and the Fermi Gamma-ray Space Telescope, have found even more decisive evidence of excess positrons.

Unfortunately for dark matter hunters, the specifics of the probes’ observations do not match up well with theories that predict cosmic rays resulting from dark matter annihilation. Along with electrons and positrons, dark matter annihilation should produce other signals like extra antiprotons (protons’ antimatter siblings), gamma rays and radio waves. But detectors have found no evidence of any of those signals.

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Wait there is hope check this out.

DENVER — Ultracold crystals designed to catch particles of dark matter deep underground have come up with three potential detections, physicists reported April 13 at a meeting of the American Physical Society.

The researchers do not have enough evidence to say they have discovered dark matter particles, but the finding qualifies as a rare clue in the frustrating quest to understand the universe’s most elusive substance.

Theoretical physicists have put forth some ideas for particles that might constitute dark matter, including one called a weakly interacting massive particle, or WIMP. The hope is that even though dark matter doesn’t often interact with regular matter, WIMPs may do so occasionally.

The experiment that made the newly reported detections is designed to pick up the signal of a WIMP as Earth passes through the galaxy’s sea of dark matter. The Cryogenic Dark Matter Search consists of a network of silicon and germanium crystals cooled to near absolute zero. It sits in the Soudan Underground Laboratory in Minnesota, a former iron mine more than 700 meters beneath the surface.

If WIMPs exist, one should very occasionally slam into the nucleus of a silicon or germanium atom, causing a release of energy and a detectable vibration in the crystal. The hundreds of meters of earth above the experiment prevent other particles, such as protons and neutrons, from reaching the crystals and triggering a false positive.

McCarthy reported that between July 2007 and September 2008, two of the experiment’s 11 silicon crystal detectors picked up three signals consistent with those expected from WIMP interactions. If the signals were caused by WIMPs, McCarthy estimates the dark matter particle would weigh in at about 10 times the mass of the proton, well below many theoretical estimates. The results also appear in a paper posted online April 15 at arXiv.org.

While the crystals’ underground setup provides plenty of shielding, some non-WIMP particles, such as electrons on the crystals’ surface, can cloud the results. The CDMS researchers say it’s extremely unlikely that three events would show up from non-WIMP sources.

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There is a lot of research going on in and off the world looking for the key to explaining or at least identifying Dark Matter. Some call it DAMA others call it WIMPs (bad name) anyway there are those out there trying to understand and explain the universe and even though I do not understand everything they are doing by a long shot I get the gist of it.

Dark matter is what confounds so many it does for myself the idea that is there but having no clue as to what it is made up of. You could become philosophical about it.

I wish they would stop calling Dark Matter invisible, its transparent...

Originally posted by Shdak
I wish they would stop calling Dark Matter invisible, its transparent...

"We don't know if it even exists. Its fully theoretical, and seems unsupported by evidence. But we can definitively say that it isn't "invisible". Its "transparent".

Originally posted by bigfatfurrytexan

Originally posted by Shdak
I wish they would stop calling Dark Matter invisible, its transparent...

"We don't know if it even exists. Its fully theoretical, and seems unsupported by evidence.

The second part is not true, there is lots of astrophysical evidence there is something with independent degrees of freedom distinct from normal matter which interacts gravitationally.

There isn't any satisfactory microscopic explanation though. LHC seems to have ruled out the simplest supersymmetry theories which most people had hoped would provide the solution.

reply to post by mbkennel


There is evidence for SOMETHING. But when I read the OP article it talks about how the evidence doesn't support what is being called dark matter.

Originally posted by mbkennel
There isn't any satisfactory microscopic explanation though. LHC seems to have ruled out the simplest supersymmetry theories which most people had hoped would provide the solution.

The LHC hasn't ruled out much in the way of supersymmetry. Some classes of models have been ruled out, but 99.999% of the reasonable supersymmetric theories have not been ruled out. There're still plenty of chances for supersymmetry to solve this problem.

And yeah, there is tons of evidence for dark matter. It definitely exists. We just don't know its microscopic properties yet.

reply to post by Moduli



Being a lowly bean counter, would you mind reconciling your statement that dark matter definitely exists with this statement from the OP

Unfortunately for dark matter hunters, the specifics of the probes’ observations do not match up well with theories that predict cosmic rays resulting from dark matter annihilation. Along with electrons and positrons, dark matter annihilation should produce other signals like extra antiprotons (protons’ antimatter siblings), gamma rays and radio waves. But detectors have found no evidence of any of those signals.

While I can read and comprehend the above, I also am painfully aware that writers tend to water things down to such a degree that it can be wholly untruthful.

Originally posted by bigfatfurrytexan
Being a lowly bean counter, would you mind reconciling your statement that dark matter definitely exists with this statement from the OP

No direct detection experiment has (so far) produced statistically significant evidence for any particular model of dark matter. A few have been ruled out, but not many. The quote was probably referring to the fact that some not-statistically-significant-result from some experiments don't match other non-statistically-significant-results from this one. Which is an unsurprising state of affairs.

reply to post by bigfatfurrytexan


I think he means evidence as in there has to be something there generating those gravitational forces.

I think most of science agrees on that at this point. He isn't saying we have been able to identify what it is composed of just that it has to be there.

reply to post by Grimpachi


Ahh....got it.

Yes. I would believe that it is known that there is a ghost in the machine. But, like a real ghost, we may stand little to no chance of detecting it.

Speaking of which....when will science seek to explain those kinds of Earthly phenomena?