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Hunting for dark matter — inside the Earth

The Cryogenic Dark Matter Search is one of the most sensitive efforts to track down dark matter particles. But the best dark-matter detector may be Earth itself, a new study suggests.
The Cryogenic Dark Matter Search is one of the most sensitive efforts to track down dark matter particles. But the best dark-matter detector may be Earth itself, a new study suggests.
(Image: © SuperCDMS/Slac National Accelerator Laboratory)

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights

Dark matter is a hypothetical component to our universe, used to explain many strange behaviors of stars and galaxies. 

Despite the almost overwhelming evidence that dark matter does indeed exist, we still don't know what it's made of. Detectors scattered around the world have been operating for decades, trying to catch the faint trace of a passing dark matter particle, but to no avail. A new paper offers an alternative approach: dig deep.

Related: The 11 biggest unanswered questions about dark matter

We know that dark matter exists through a variety of astronomical observations. Stars are orbiting the centers of their galaxies too fast. Galaxies are whizzing around inside clusters too quickly. Massive structures in the universe are appearing too early.

As far as we can tell, there is much more to the cosmos than meets the eye — there is some form of matter that is entirely invisible to us. Whatever the dark matter is, it's a new kind of particle that doesn't interact with light, which means it doesn't emit, absorb, reflect or refract electromagnetic radiation. Which means we can't see it. Which makes it dark.

So far, the only way we know dark matter exists is through gravity. Despite its invisibility superpower, dark matter still has mass, which means it can tug and shape on the biggest objects in the universe, revealing its presence through the motion of the more luminous stars and galaxies.

On the other end of the scale, particle physicists have been concocting new particles as consequences for new theories of physics, and some of them fit the bill for what the dark matter could be. The most promising candidate is a particle known as a WIMP: a weakly interacting massive particle.

The "weakly interacting" part doesn't just mean the particle is feeble: it means that the dark matter does occasionally interact with normal matter through the weak nuclear force. But as the name suggests, the weak nuclear force isn't the strongest, and it has very short range, making these interactions incredibly rare.

Related: Dark matter and dark energy: the mystery explained (infographic)

Buried clues

But "rare" doesn't mean "never." It's thought that billions — even trillions — of dark matter particles are swimming through you right now. But since the dark matter hardly notices normal matter, and vice versa, you simply don't feel it. You have to go out to big scales before you start to see its gravitational effects.

Still, rarely (exactly how rarely is not known yet), a dark matter particle goes rogue and interacts with a particle of normal matter through the weak nuclear force. This involves a transfer of energy (i.e., the dark matter particle kicks the normal particle), sending the normal matter flying, something that we can, in principle at least, detect.

But since it's so rare and so weak, our detection attempts haven't proven fruitful. We need big detectors that take up a lot of volume (since the interactions are so rare, it's either build a giant detector or wait for hundreds of years to get lucky). What's more, we have to bury these detectors deep underground, the deepest going 1.2 miles (2 kilometers) below the surface. This is because there's a lot of subatomic nuisance going on: other high-energy particles, like neutrinos and cosmic rays, cause similar kicks, and we need to use lots of rock to absorb them before they hit the detector, ensuring that if we do see a signal, it's more likely to be caused by dark matter.

And so far, after decades of building ever-larger detectors and watching carefully, we haven't found squat.

Read more: "Searching for Dark Matter with Paleo-Detectors"

Fossil evidence

There's a limit to how big we can make a dark matter detector, based solely on engineering and cost constraints. But thankfully, according to a new paper recently appearing on the online preprint site arXiv, there's a gigantic dark matter detector that's been collecting data for millions of years. 

And it's right under our feet.

The crust of the Earth itself serves as a massive dark matter detector. When stray dark matter particles interact with normal matter inside a rock, a proton or neutron can get knocked loose, changing the chemical composition of the rock in the vicinity of the impact site. This can potentially even send the particle flying, leaving behind a microscopic scar.

Even better, deep digs have access to portions of the Earth's crust over twice as deep as our current dark matter detectors, promising results even freer of confusion from cosmic rays and other nuisance particles. And since rocks stay as rocks for millions, and even hundreds of millions, of years, they've been recording dark matter interactions for all that time, far longer than we can ever hope to access in the lifetimes of our experiments.

So it's pretty simple: dig up a bunch of rock (preferably something pure, so it's easy to analyze) and look it over with a fine-tooth microscopic comb, looking for any signs of subatomic violence.

There is one catch, however. Earth rocks naturally contain some radioactive elements, and radioactive decays will give rise to similar features. To solve this, the researchers suggest digging into oceanic crust, which is much more pure than the stuff that builds continents. With this in hand, the researchers predict that we could have a super-detector within easy reach: even a mere kilogram of rock would beat the sensitivity of the world's current best detectors. 

We just have to dig in.

You can listen to the Ask A Spaceman podcast on iTunes, and on the Web at http://www.askaspaceman.com. Ask your own question on Twitter using #AskASpaceman, or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter. Follow us on Twitter @Spacedotcom or Facebook

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  • dfjchem721
    Here is one for all the dark matter freaks out there:

    Sorry, off direct topic of article---

    According to Wikipedia, Dragonfly 44 is a galaxy which appears to be made up almost entirely of dark matter, with a mass approximating that of the Milky Way, or so the story goes. It is an extreme outlier as it appears, so far at least, to be the only one of its kind. A massive dark matter galaxy. If dark matter is so prevalent, shouldn't we see more galaxies like this, instead of just one? We have been looking in a lot of places. Not a cosmologist, so take it easy on me........

    I am dying for an answer on this one! Maybe Dragonfly 44 is not a galaxy of dark matter, but simply some odd-ball. Stranger things have been known, maybe. Or not.
    Reply
  • Bigrob215275
    I find the current state of theories absurd. After looking intensely for decades AND FINDING NOTHING Physicists act like spoiled little children screaming and acting out that they are right and it doesn't matter what anyone says they are right because they say it. Instead, they should admit the shortcomings of the theories and keep looking and keep thinking up new theories that may explain the Universe. They have explained only 4-6% of the Universe and they should be humble in their approaches and presentation of ideas. I am a Christian and I believe in GOD because JESUS came to Earth for my sins. There is far more PROOF for my position than there is for DARK ENERGY, DARK MATTER, or DARK FLOW none of which are explained or discovered in any real sense of observations.
    Reply
  • dfjchem721
    Physicists get a lot of things right. Hiroshima and Nagasaki are dramatic proof of this. But those bombs were made less than ten years after all the high brows in physics claimed that fission of heavy nuclei was impossible - meaning such bombs were impossible. It took a lowly radio-chemist (Otto Hahn) to prove they were wrong. They were very certain about this before Otto set them straight. Apparently he had not heard that fission was impossible!

    Einstein did not believe black holes (BHs) were possible, but he was wrong about that one. Neither did Hawking at first, but his is one of the biggest names in BHs. But I will agree that these people seem to take a lot for granted, and that their big brains are always right. They have big egos that need feeding, just like a black hole. And being wrong in science is the worst fate. We are only human, after all. Cosmological observations strongly suggest dark matter and dark energy, but it remains possible that some of this stuff is simply beyond our comprehension.
    Reply
  • rod
    Dark matter apparently does not show up in spectroscopy studies of starlight. From what I understand about stellar spectrums, neither do 11 dimensions that the light traveled through using string theory or a QFT using inflation cosmology. Dark matter is an interpretation of various motions, initially in spiral galaxies and rotation curves like Zwicky in the 1930s and others later. So far, reporting DM on the Periodic Table remains elusive :)
    Reply
  • dfjchem721
    One comment on theories. When one uses theories to support other theories, they sometimes get so far out in left field they are no longer in the ball park.

    Dark matter appears to have some support in the Standard Model however. I was reading about how quarks and gluons can form in the Big Bang and not become neutrons and protons, but some unique form of matter. The only place on the Periodic Table I have found reference to dark matter relates to molecular hydrogen, which is very difficult to detect and some believe its quantity is vastly underestimated in the universe, particularly as it relates to "halo objects". But I don't recall anyone ever suggesting that the Periodic Table offers the all-inclusive character of atoms and their properties. So much is unknown.

    They could be wrong about H2, or another Otto Hahn. Time will tell, or not............
    Reply
  • rod
    Testing for dark matter (DM) remains elusive and not confirmed as to what DM is (assuming DM is real). The solar system does not show DM like observing the Galilean moons and events at Jupiter or Mars moving through Sagittarius this morning when I viewed with the telescope, passing Jupiter's position in Sagittarius. New Horizons is another example, DM is not used to explain New Horizons motion or predict when it flew by Pluto. Astronomy is chasing DM since the 1930s and still looking. If DM is not real, the cosmology department falls apart, no structure can form after the Big Bang :)
    Reply
  • dfjchem721
    Since the Big Bang theory has (at least) one major flaw (baryon asymmetry), it is imprudent to assume the rest of it is absolute. If you have one proven flaw in a theory, you have empirical evidence that other flaws could exist. Many structures made up of quarks and gluons could have formed during the BB, along with protons, neutrons, etc. and all the other things we do not know about.

    Again, much is unknown. Almost certainly much more than is known.
    Reply
  • Torbjorn Larsson
    Admin said:
    Detectors scattered around the world have been operating for decades, trying to catch the faint trace of a passing dark matter particle, but to no avail. A new paper offers an alternative approach: dig deep.

    Hunting for dark matter — inside the Earth : Read more

    After several experiments failed to find WIMPs at the natural scale - a bit higher in mass that baryon matter - at the same time inflation did find consensus evidence, I lost confidence in supersymmetry and so string theory. Same as now is happening to axions/axion -like particles that are natural to string extensions to the baryon matter sector. There is now no compelling reason to think dark matter or axions are anything as complicated as and/or tied to baryon matter.

    But these things will test some of the remainder parameter space, so we'll see.
    Reply
  • Torbjorn Larsson
    dfjchem721 said:

    Here is one for all the dark matter freaks out there:

    According to Wikipedia, Dragonfly 44 is a galaxy which appears to be made up almost entirely of dark matter, with a mass approximating that of the Milky Way, or so the story goes. It is an extreme outlier as it appears, so far at least, to be the only one of its kind. A massive dark matter galaxy. If dark matter is so prevalent, shouldn't we see more galaxies like this, instead of just one? We have been looking in a lot of places. Not a cosmologist, so take it easy on me........



    I am dying for an answer on this one! Maybe Dragonfly 44 is not a galaxy of dark matter, but simply some odd-ball. Stranger things have been known, maybe. Or not.

    Dark matter is part of current Lambda-Cold Dark Matter cosmology, it says so in the name. Dark matter is crucial to star and galaxy formation, else we wouldn't see much of it, since non-dark matter wouldn't suffice to clump itself. So the generic, average result is that those two are seen together in galaxies as a whole.

    There are differences between the two types of matter obviously, and one famous consequence is that galaxy collisions may separate the two (since non-dark matter likes to interact and will tend to lump easier).


    https://en.wikipedia.org/wiki/Bullet_Cluster ]
    "Because we've never directly detected whatever particle might be responsible for it, many people — experts and laypersons alike — remain skeptical of its existence. But if our Universe didn't have any dark matter, it would be a very different place. Here's how."


    "On larger cosmic scales, there would be dramatically less structure overall. In a Universe without dark matter, there is no unseen "skeleton" to the cosmic web; instead, structure forms based on the strength of normal matter alone. This means that instead of a cosmic web, where you wind up with galaxies dotting the filaments that connect the great clusters of the Universe together, you'd just wind up with isolated islands of mid-sized galaxies, with not much else.

    Sure, some galaxies would still group and cluster together, but there would be far less of them that do so in a Universe without dark matter."

    https://www.forbes.com/sites/startswithabang/2020/02/14/the-universe-would-be-very-different-without-dark-matter/#17f6b10237ea ]
    "We can tell that dark matter exists and even infer some of its properties by observing how it affects the matter and light we can observe, particularly in large-scale astrophysical environments. But the fact that dark matter has eluded direct, laboratory detection thus far means that a number of its properties remain open questions. Here are five things we know about dark matter, along with five that we don't, as we probe the limits of our scientific frontiers."

    "4.) Dark matter's effects are most dominant, on average, in the smallest galaxies of all.

    This one's a little bit counterintuitive, but has been observationally validated practically everywhere we look. Under the laws of gravitation, all forms of matter are treated equally. But the other forces, like nuclear and electromagnetic forces, only affect normal matter. When a large burst of star formation takes place in a galaxy, all of that radiation simply passes through the dark matter, but it can collide with and be absorbed by the normal matter.

    This means that if your galaxy is low enough in mass overall, that normal matter can be expelled by intense episodes of star formation. The smaller and lower-in-mass your galaxy is, the greater the amount of normal matter that will be expelled, while all the dark matter will remain. In the most striking examples of all, dwarf galaxies Segue 1 and Segue 3, both satellites of the Milky Way, contain only a few hundred stars, but some 600,000 solar masses of material overall. The dark matter-to-normal matter ratio is approximately 1000-to-1, as opposed to 5-to-1 in most large-scale structures."

    https://www.forbes.com/sites/startswithabang/2020/03/13/5-thing-we-know-about-dark-matter-and-5-we-dont/#784841a736f8 ]
    Reply
  • Torbjorn Larsson
    Bigrob215275 said:
    I find the current state of theories absurd. After looking intensely for decades AND FINDING NOTHING Physicists act like spoiled little children screaming and acting out that they are right and it doesn't matter what anyone says they are right because they say it. Instead, they should admit the shortcomings of the theories and keep looking and keep thinking up new theories that may explain the Universe. They have explained only 4-6% of the Universe and they should be humble in their approaches and presentation of ideas. I am a Christian and I believe in GOD because JESUS came to Earth for my sins. There is far more PROOF for my position than there is for DARK ENERGY, DARK MATTER, or DARK FLOW none of which are explained or discovered in any real sense of observations.

    I find the current state of trolls absurd. After not looking at all for decades AND FINDING NOTHING Trolls act like spoiled little children screaming and acting out that they are right and it doesn't matter what anyone says they are right because they say it.
    Reply