Guide to the Cosmos

Making the Wonders of our Universe Accessible to Everyone.

 Newsletter: Dark Matter & Golden DNA

Dark matter is the greatest enigma in physics, astronomy, and cosmology. Recently, some new possible sightings were reported and a remarkable new technique, based on DNA, has been proposed.


Starting 80 years ago, scientists began seeing hints of vast clouds of invisible matter surrounding galaxies and galaxy clusters. This so-called dark matter is elusive and nearly inert. It is not made of normal atoms nor normal particles; it emits no light and casts no shadow, but exerts a dominating gravitational force on all the normal matter we do see.


The Bullet Cluster reveals much about dark matter. We see below the aftermath of the collision of two galaxy clusters containing 1000 trillion stars. The two purple halos are dark matter, whose presence is inferred from its gravitational impact.  Each halo is undisturbed, demonstrating that dark matter doesn’t interact with galaxies, with ionized gas (the pink clouds), or even with the other dark matter halo.



In the last decade, NASA’s WMAP satellite firmly established that there is 5 times more dark matter than normal matter in our universe.


The leading guess, and without any direct evidence this truly is a guess, is that dark matter consists of, as yet undetected, weakly interacting, massive particles (“WIMPs”). WIMPs are thought to be their own antiparticles (as are photons), hence if two collide they can annihilate.


For several decades, experimental physicists have searched for WIMPs, with little success. At least three experiments claim to have seen hints of WIMPs, while at least three others failed to confirm their findings.


Two new possible sightings have just been reported, but it’s still iffy.


About 18 months ago, the Alpha Magnetic Spectrometer (AMS) was mounted on the exterior of the International Space Station, at a cost of $2 billion. The AMS analyzes cosmic rays, measuring the energy, electric charge, and type of each particle. Of special interest are positrons, the antimatter version of the electron. (A normal atom has a tiny nucleus containing protons and neutrons, surrounded by a much larger cloud of electrons.) The AMS detected an unusually large number of positrons at energies corresponding to masses that span the range from a carbon atom to a uranium atom. The AMS research team suggests the excess positrons come from WIMP annihilations. Maybe. But since the annihilations weren’t observed, the evidence is indirect. Scientists will have to be convinced that no other cosmic phenomena could have created these positrons.


While some search in outer space, others hunt in old mines, a mile underground. The Cryogenic Dark Matter Search (“CDMS”) group reported three possible events: germanium nuclei recoiling sharply, as if from a collision with an unseen, heavy particle. Again, the purported WIMP interaction is not directly observed. Only the slight energy of a single recoiling nucleus is detected. CDMS will continue collecting data, and must convince other scientists that these small energy signals don’t come from any conventional source.


Meanwhile, a team, including astrophysicist Katherine Freese from the University of Michigan and geneticist George Church from Harvard, have proposed a remarkably novel approach. They plan to construct a WIMP detector consisting of trillions of strands of DNA and thin gold sheets. Each DNA strand would have the same genetic code and would be 10,000 nucleotides long. Each strand hangs vertically, with one end attached to a horizontal gold sheet. The DNA strands will be spaced 1 nanometer (a billionth of a meter) apart. That spacing is about 100,000 times smaller than the diameter of an average human hair.


The detection scheme is ingenious. An incoming WIMP will occasionally hit a gold nucleus and knock it downward out of the sheet, as sketched below. The gold nucleus, being heavy and ionized, severs DNA strands as it passes through the detector. (They plan to use single-stranded DNA, which is more fragile.) The severed DNA pieces drop to a collector, and are analyzed to determine the length of each piece to a precision of 1 nanometer. Measuring the gold nuclei trajectories reveals the directions of the incoming WIMPs, as never before determined.



This detection system is planned to have 10,000 trillion resolution elements, called “voxels.” The team claims they will be able to determine exactly which voxels each gold nucleus traverses. That’s 1000 times greater precision than any conventional device.


Wonderful progress is sometimes achieved when scientists from two apparently unrelated fields join forces. Such was the case when particle physicists and cosmologists began collaborating.


We think dark matter completely encircles us. As the Sun orbits the Milky Way, we should be moving through an ocean of WIMPs, or equivalently, a current of WIMPs should be flowing by us. And as Earth orbits the Sun, the WIMP current would be flowing by Earth at different speeds depending on the time of year; one month we would be heading into the current, and 6 months later we would be heading the opposite way. Thus, the WIMP signals should vary throughout the calendar year, which helps separate that signal from the background of conventional interactions, such as cosmic ray impacts. All this assumes we have some understanding of dark matter.


Hunters of the dark side of our universe are enthusiastic about finally catching WIMPs within the next few years. May the force be with them.


Best Regards,
May 29, 2013


Note: Previous newsletters can be found on my website.


Below are my latest eBooks in the Everyone’s Guide Series.  They are all available for Kindle on and for Nook on Barnes&, for $2.99 each


(Click on each cover for more information.)




Dr. Robert L. Piccioni


Author of

"Everyone's Guide to Atoms, Einstein & the Universe"

"Can Life be Merely an Accident?"


"A World Without Einstein"


Guide to the Cosmos