Is dark matter about to be discovered?

-April 25, 2015

The evidence that dark matter exists is pretty simple: the visible mass of galaxies is too small to account for how tightly gravity holds them together. In other words, if the visible mass is all there is, then galaxies should be flying apart much faster than they do. The obvious solution: there must be some invisible matter whose mass holds galaxies together through its gravitational attraction.

The difference between the white and red curves shows the discrepancy between the measured attraction of galaxy components and that calculated from the gravitational attraction of visible objects. Source:

Another remedy would be to modify Newton’s law of gravity and some smart people who understand the nuances of the problem have modifications ready to roll, but Newton’s venerable law has survived too many experimental probes to be tossed aside so quickly. An essential tenet of the scientific method, usually called Occam’s razor, phrased by Newton as his first rule of reasoning: “We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances.” So before we toss aside a theory that works, we search for what we might have missed.

Dark matter is probably WIMPy

If the excess matter were composed of neutrons, protons, electrons, and their interactions—you know, like normal stuff—then it would interact with the radiation from stars and we could see it. In other words, being “dark” means that it probably doesn’t interact through the electromagnetic force. We talked about the different forces—gravity, weak nuclear, strong nuclear, and electromagnetic—in “The quirks of quarks."

Since the evidence for dark matter comes from the gravitational attractions that hold galaxies together, we know that it must interact gravitationally. If dark matter interacted through the strong nuclear force, we’d see big, loud, screaming evidence in the form of x-rays and gamma rays; there’s nothing subtle about the strong force. So process of elimination leads to the reasonable assumption that dark matter interacts through just the gravitational and weak nuclear forces. In other words, it seems that dark matter is WIMPy.

WIMP stands for “weakly interacting massive particle.” Massive means that it interacts through the gravitational force and weakly interacting means that it interacts through the weak nuclear force. In “Measure of neutrinos, nature’s most elusive particles” we talked neutrinos and their interactions.

Experiments designed to detect dark matter have a lot in common with neutrino detection experiments. Dedicated dark matter experiments need to be well shielded from the noisy background of normal matter, so most are located deep underground. They use large volumes of pure targets embedded with detectors to see the slightest trace of an interaction. Some operate at temperatures just above absolute zero to quiet electrical noise.

Several experiments in the last few years have seen hints of dark matter. The hints tantalize experimentalists but fall short of the evidence required for discovery. We’ll dig into the dark matter detection technology and engineering next time—and you’re going to love how clever it is—but first, let me summarize the hints.

  • CDMS (cryogenic dark matter search), an experiment 700 m deep in a Minnesota mine, has seen evidence that a few dark matter particles have passed through their detector. The particles’ masses are around 8 GeV (billion electron volts, roughly the same as 8 protons).
  • CoGENT (Coherent GErmanium Neutrino Technology—the acronyms in this racket get a little convoluted), a neighbor of CDMS down in that Minnesota mine, has also seen signals of particles in excess of the background level at about the same mass as CDMS.

Dark matter experiments take place at Soudan Mine State Park in Minnesota. Source: Gallivant

  • The DAMA/LIBRA (DArk MAtter/Large sodium Iodide Bulk for RAre processes) experiment collects data 1400 m below Gran Sasso Mountain in Italy. For years DAMA/LIBRA has been seeing a strong signal right around 10 GeV in synch with earth’s orbit around the sun. The seasonal waxing and waning of the signal could indicate a patch of denser dark matter on one side of the sun compared to the other.

The DAMA/LIBRA experiment collects data 1400 m below Gran Sasso Mountain in Italy.

With this corroborating pileup of evidence, why haven’t any dark matter hunters been invited to Stockholm or been seen hanging around Porsche dealerships?

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