Particle Physics


Journey Into the Dark Realm

After nearly a century of observations, astronomers have concluded that the type of matter that makes up you and me amounts to just a scant 5% of the recipe of the universe. A ghostly form of matter called dark matter is five times more common than our familiar atoms. True to its name, dark matter emits no light; we “see” it only indirectly, by measuring its gravitational pull on ordinary atoms. So how do we know it’s really there? To be sure, we need to detect dark matter directly.

Flickr/luxdarkmatter under a creative commons license

Physicists have been searching for dark matter particles for decades now. Some experiments seem to have caught them while other, equally powerful experiments have failed to find any evidence for dark matter. Most recently, the ultra sensitive LUX detector, a vat of liquid xenon buried in a mile-deep underground lab, found no evidence for dark matter and ruled out earlier measurements that had reported hints of a signal. Does this mean one or more of these results is wrong? Not necessarily. There are ways for both the LUX measurement and earlier measurements to be true, but this requires that dark matter and ordinary matter interact with each other in very specific, unexpected ways. Scientists are exploring these possibilities.

At the same time, physicists are beginning to think a bit more creatively. Until now, scientists looking for dark matter have imagined that dark matter is very simple. Specifically they imagine that there is just single type of dark matter particle: electrically neutral, experiencing only the weak and gravitational forces and with a mass 10-1000 times that of a proton. This model is popular because it is simple. On the other hand, the universe is not obliged to honor our definition of simplicity.

Suppose someone was studying the behavior of ordinary matter using only gravity as a probe. They’d no doubt construct a simple model of matter as a particle that was something like a neutron. However, we know that our world is very complex, that the neutron is just one member of the particle zoo and that these particles can come together in all sorts of interesting ways. Scientists are beginning to wonder if maybe dark matter might be similar.

Perhaps dark matter isn’t just one particle but a diverse realm of dark matter particles that experience forces that don’t affect ordinary matter. These dark matter particles might interact fairly strongly with each other, but only weakly with ordinary matter. With little experimental evidence to guide them, theoretical physicists are allowed to speculate fairly freely, although there are some constraints imposed by astronomical observations.

One idea postulates a dark equivalent to electrical charge called “dark charge.” Just as ordinary electrons and positrons (antimatter electrons) can interact with each other and emit photons, it is possible that particles carrying dark charge can interact and produce dark photons.

It is crucial to remember that dark charge, if it exists, does not interact with ordinary matter except by way of gravity and maybe the familiar weak force. A dark matter particle carrying dark charge and a familiar particle carrying electrical charge would pass by one another without so much as a “how do you do?”

If a complicated dark sector exists, we can see it only if there is a particle that interacts with both ordinary matter and dark matter. If we could create such a messenger particle and allow it to interact with astronomical dark matter or (more likely) decay into dark matter particles, we might be able to detect it at particle accelerators like the LHC. But there’s a catch: The experimental signature would be “missing” energy in some collisions as the energy flowed into what physicists call the dark sector, the enigmatic realm of dark matter and dark energy. Given that disappearing energy is a fairly common feature of particle collisions (e.g. when neutrinos are created), it would be tricky to pin it on the creation of dark matter messenger particles. But by measuring the distribution of “missing” energy in LHC collisions and comparing it to the predictions of known physics and theoretical models of dark matter particles, it might be possible to catch a glimpse of the dark sector.

Of course, missing energy is just one possible signature of a complicated dark sector. Another possibility invokes the principle of supersymmetry, which postulates that every known fundamental subatomic particle has a (so far undiscovered) cousin with a different quantum spin. Were the LHC to create these theoretical supersymmetric particles in a collision, they would decay into low-mass supersymmetric particles capable of interacting with the complex dark matter sector. After another cascade of decays, a dark matter particle could emit a messenger particle that “sees” both dark matter and ordinary matter and then decay in turn into a matter-antimatter particle pair that could be picked out in the collider data. Because this scenario postulates both supersymmetry and complex dark matter, it is even more of a jump into the unknown. But given that we don’t understand a lot of the universe, sometimes wild ideas are required. As Niels Bohr once quipped to Wolfgang Pauli, “We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough.”

So far, physicists have not found evidence for a complex dark sector, but the search has just begun. Ordinary matter is complex, so it seems very reasonable that the dark sector should be, too. Over the next several years, theorists will begin to flesh out a myriad of dark possibilities, including possibly even dark atoms, just in time for the LHC to turn back on and see if the data supports these interesting ideas.

Go Deeper
Author’s suggestions for further reading

arXiv: Dark Sectors and New, Light, Weakly-Coupled Particles
A technical paper summarizing the motivation for and possible tests of dark sector theory.

Preposterous Universe: More Messy Dark Matter
Astrophysicist Sean Carroll blogs about the possibility that dark matter is more “interesting” than we thought.

Sanford Underground Research Facility: First results from LUX experiment in South Dakota
A press release outlining the results of the LUX experiment’s first, three-month-long search for dark matter.

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Don Lincoln

    Don Lincoln is a senior experimental particle physicist at Fermi National Accelerator Laboratory and an adjunct professor at the University of Notre Dame. He splits his research time between Fermilab and the CERN laboratory, just outside Geneva, Switzerland. He has coauthored more than 500 scientific papers on subjects from microscopic black holes and extra dimensions to the elusive Higgs boson. When Don isn’t doing physics research, he spends his time sharing the fantastic world of science with anyone who will listen. He has given public lectures on three continents and has authored many magazine articles, YouTube videos and columns in the online periodical Fermilab Today. His most recent book "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind" tells the tale of the Large Hadron Collider, the physics and the technology required to make it all work, and the human stories behind the hunt for the Higgs boson.

    • Chuckster

      The alternate universes theory still sounds to many like a B-movie plot device.
      However, even doubters know that, as science advances, things that once drew laughter and rueful headshaking may be punching holes in steel with nothing but light in a laboratry near you, or storing hundreds of gigabytes of data in a handheld device wirelessly connected to a worlwide web. The whole subject of “branes” is so esoteric for us monkey boys that relatively few humans have the knowledge to even discuss it. If it’s fairytales and overreaching, it takes a physicist to prove it so. I, too, have wondered if the unaccountable gravity out there originated “elsewhere”. Maybe elsewhere is just a place holder, like “dark matter”, until we can learn more.

      Remember your Stanislaw Lem : “Physics, my friend, is a narrow path drawn across a gulf that the human imagination cannot grasp. It is a set of answers to certain questions that we put to the world, and the world supplies answers on the condition that we will not then ask it other questions, questions shouted out by common sense. And common sense? It is that which is understood by an intelligence using senses no different than those of a baboon.”

    • Donald Duck

      I believe the Universe has a structure like a galaxy and dead black holes are the center of most matter including dark matter and planets including Earth.

      • William F. Nappi

        That’s deep.

        • Zippy

          Hi Bill,

          I think Emily Post is still in print. I believe that Dale Carnegie’s book, “How to Win Friends and Influence People” is also still available!



          • William F. Nappi

            Zippy, Napolean Hill’s “Think and Grow Rich is an interesting crock of sh*t. When was the last time you read ‘Horton Hears a Who”?

            I suppose all this is relevant.

          • William F. Nappi

            Also Zippy, I read Dale’s book (like 30 years ago.) I think it’s pretty gay. There’s a video that never went to theaters as far as I know. “How To Lose Friends and Alienate People,” with Kirsten Dunst, Meg (what’s her name from Transformers), Jeff Bridges and one other British guy who’s the star.

            Influence that.

    • Renee Cole

      A scholar in the scientific field of the universe, I am not, but mysteries are just questions unanswered. I find dark matter to be very intriguing and there will come a day, just as Newton’s Theory of Gravity, be an answer to this mystery. I enjoy reading all the “What ifs”. I hope I am still around when the mystery is one day solved.


      i want to get your view on

      1. Dark matter is also made up of dark atom.
      2. Dark matter is more concentrated in the centre of galaxies but it is not the warehouse of them, it is the PRODUCTION HOUSE.
      3. Dark matter do not play a major role in galaxy formation, it is a part of GALAXY.
      4. Earth has less dark matter but it is not due to very large distance from galactic core.
      5. When I say smaller dark atom it means size not its concentration.
      6. Dark matter do not plays a direct role in star formation but its alignment will help in star formation just away the galactic core in DISC not in HALO.

      Yes, my assertions are appearing as pure speculation but all my comments (very short) on black holes, dark matter, supernova, gravity etc.( are on the basis of dark matter & dark energy.

      I can explain the cause of lots of phenomenon on which our scientists are working. I can give the evidences of my speculation by explaining the causes of gravity, formation stars, formation of planets, formation of moon, formation of dark atom, formation of dark energy, how dark atom are continuously interacting with the environment, why dark matter is more near galactic core?, what is the destination of dark atom formed in galactic core?, formation of electron ring around the earth, why comet is not the source of life on earth, why neutrino found more near Antarctica, why a molded metal object remain in the same shape ….. these are only the few.

      My work is important because I am thinking out of box, I am looking the Universe from another window (of Dark Matter & Dark Energy) while our scientific window is different but on many topic we are drawing the same picture of universe.

    • zZz

      interesting! Uncovering the world of the unseen. hope you wont find horrible things!

    • Kurtis

      I find it curious that the existence of dark matter is posited because there doesn’t appear to be enough visible mass in the galaxies to keep their spiral arms from hurling them apart while the existence of dark energy is posited because the expansion of the universe appears to be accelerating and can’t be the consequence of momentum from the original big bang. We can’t find direct evidence for the existence of either of these but have made them up to account for observations that don’t jive with our understanding of the universe and our galaxy. Dark matter doesn’t seem to operate between galaxies and dark energy doesn’t seem to operate within them. Maybe dark matter does not exist and our observations or understanding of gravity are flawed as it applies to our galaxy. The same may be said of the expansion of the universe. Both theories strike me as being somewhat lazy and ad hoc ways of accounting for discrepancies between observations and our present understanding of gravity and momentum. Maybe our understanding of these phenomenon are not complete and both of these forces have more complex behaviors when operating at different scales. I’m no physicist, but I think we might be barking up the wrong tree.

      • phish

        EXCELLENT point

      • Actually Kurtis, I must disagree.

        Dark matter seems to exist in all (or almost all) galaxies and is necessary to describe the motion of large clusters of galaxies. Further, dark matter is necessary to explain gravitational lensing on large scales. If you Google “Bullet Cluster,” you will find the most compelling reason why scientists think dark matter is right.

        Now you are correct that dark matter hasn’t been found in the laboratory and we’re looking for it very hard. There are many experiments working on just that.

        Scientists don’t >>believe<< in dark matter or dark energy. They just are the most economical hypotheses to describe the data we see. We await confirmation and, in the meantime, we consider other explanations, including modifications of Newton's laws of motion. (Google MOND if you're interested, but the Bullet cluster has done a good job of clobbering MOND as a single answer to the observations that lead most scientists to acknowledge the strength of the dark matter hypothesis.)

        • Kurtis

          Thanks for your reply Don. I googled “Bullet Cluster” as per your recommendation and it did not take long in reading about this to know that this subject is beyond my depth not having a back ground in physics or math. However, I did not realize, until now, that there was a competing theory for dark matter nor did I fully understand the nature of the discrepancy between the observations and the laws of motion. It is odd that all stars rotate at the same velocity regardless of the distance from the center. In one article, there was a suggestion of the possibility of creating an experiment that could test the MOND theory within our solar system using a spinning disk. I hope that within my lifetime some of these questions will be answered with definitive proof. The nature of this dark matter must be very strange indeed because it would seem to possess some of the same properties of conventional matter, exerting gravitational influence affecting the gravitational field, but not interacting in exactly the same way. Otherwise I would think we would have dark matter suns, planets, asteroids, and meteors and I’d have to wonder why conventional matter doesn’t commingle with it since presumably gravity is gravity. Anyway, I have just enough layman’s understanding to be confused and must rely upon the astrophysicist to sort it out for me.

          • Hi…
            Matter and dark matter certainly do comingle. However dark matter, if it exists, doesn’t have electrical charge, which is why it doesn’t make atoms. (The electric charge of the protons and electrons are why atoms form.)

            On the other hand, it is possible that dark matter might experience forces that regular matter doesn’t. In fact, while the simplest proposed form of dark matter is something akin to a heavy and stable neutron, it is likely that there are many different kinds of dark matter particles.

            We will know when we know, but searching for dark matter is now a three-pronged effort. We look for cosmological dark matter by putting detectors deep in mines. We look in space to see signatures of cosmological dark matter annihilating and the annihilation products hitting Earth. Finally, we try to directly make dark matter at experiments like the Large Hadron Collider.

            It is not at all unreasonable that we may have an answer in 10 or 20 years. Maybe even five. The detectors are becoming very, very, powerful. Stay tuned!

    • Anonymoys

      Speaking of wild ideas, is it possible that the gravitational constant of the universe isn’t constant? Is it possible that the constant is higher when considering large swaths of mass in space? On a Through the Wormhole episode some scientists discovered that the weak nuclear force varied relative to the earth’s proximity to the sun…the closer the earth was in its elliptical orbit the faster the rate of decay and vice~versa. It was as if the rate of radioactive decay increased the closer that the decaying matter was to a large amount of similarly decaying matter. I wonder if the gravitational constant could also be affected this way…the larger the accretion of ordinary matter in a given space the greater the gravitational forces there may be relative to the mass. If true, this could account for at least some of the observed gravitational effects of “dark matter.”

      • Don’t believe the “Through the Wormhole” thing. That is a very sketchy measurement.

        All data is consistent with G being constant. There may be residual small differences beyond our measurement ability, but remember that dark matter is about 5x as much as ordinary matter. We’re not talking about small and subtle effects.

    • Rafal

      The fact that we are always constantly moving as well – our galaxy, our earth, down to our bodies themselves – might this skew our perception of the speed at which other bodies move?

    • William F. Nappi

      Have you ever performed the algorithms on this? That is mathematically beyond my level of skill with the information provided. (Functions??? Never mind.) The point is, what number, what value/ what was your conclusion to the question:

      “How fast is our galaxy rotating?” (In mph?) was my question as I do NOT know the answer to that question AND I know that I don’t know the answer AND I know that I would like to know the answer. (How Boolean). I keep forgetting the rate at which we (earth) rotates on it’s axis AND I then confuse what I can remember as to how fast we orbit the sun AND I wonder do we, or can we be, if observed from a far enough distance, appear to have the spin of a subatomic particle, acting as electrons in our own right orbiting a nucleus (the sun). (A collection or gathering of gazillions of composite particles in our own right(s)) AGAIN calling to consider the general theory of relativity. (Pretty sure ‘GR’.)

      Do you see where I’ve gone with this?

    • No, not much. Think about it…we are moving, it’s true. But we can look at a distant galaxy. At the same time, we can look on both sides of it. We’ll see one side moving away and one side moving towards us. If you average the two, you find the velocity at which the galaxy itself is moving. You then can subtract that off and you are left with the speed at which the galaxy is rotating.

      From that, you can see that the galaxy is rotating too fast to stay intact if it is only held in by the force of gravity from visible matter.

      See…this analysis is independent of the relative velocity of us to the distant galaxy. This is the reason we’re sure of the conclusions.

    • William F. Nappi

      I understand what you said, Rafal. I like your answer and it coincides with my positing. With that, I feel that Mr Einstein’s general (I think general) relativity is called to task. ‘Size’ – as opposed to’ OR in tandem with – distance…

    • Rafal

      Mr. Lincoln, thank you for taking the time to converse with your readership. May it lead to fruitful discussion, an advancement of a common knowledge of physics, and new discoveries in the field.

    • William F. Nappi

      “See…this analysis is independent of the relative velocity of us to
      the distant galaxy. This is the reason we’re sure of the conclusions.” Confirmation of a question I had. The rest was not relevant to my query.

    • William F. Nappi

      You mentioned the ‘velocity’ at which the galaxy is moving but the remainder of your answer reinforces to me the notion that I’m left wondering if you REALLY mean the speed AND the direction. I think perhaps I’m not the only one in need of some clarity here. You must be confused with all your knowledge.

    • You’re welcome. I can’t always pursue every conversation in depth, but it’s a poor communicator that doesn’t do his or her best to actually >>communicate<< rather than just lecture to people.

    • William F. Nappi

      You’re being more polite than me Rafal.

    • William F. Nappi

      That IS what I thought you meant, sorry about THAT. I did want to elaborate though with my take of the unknown, or that which in not known in general. Thank you for taking the time to address that, sir. Appreciate it.

    • I see where yo uare going with it. It’s a simple case of a little knowledge confusing you too much.

      You can look at a galaxy and see the velocity of stars on either side. If one is moving away from us at 10 mph, while the other is moving towards us at 4 mph, we can conclude that the galaxy is moving away from us at 7 mph. We can further conclude that one side of the galaxy is moving away at 3 mph and the other side is coming toward us at 3 mph.

      [Obviously, the numbers are wrong here, but the idea is solid. What we’re concerned about is the rotational speed of 3 mph.]

      Now to answer your numerical question, there is no single answer. Bigger galaxies have stars moving at bigger velocities. The distance depends on how far away you are from the center of the galaxy…all that stuff. For a galaxy the size of the Milky Way, the velocity of a star orbiting around the center is about 220 km/s. That’s a ballpark number, but it’s about right.

    • William F. Nappi

      Your answers are both insulting and NOT straightforward. I’m not saying you’re stupid, you’re definitely well educated and well accredited but your answer turned me from a fast admirer to just the opposite. It seems to me you don’t have the capacity to respond in laymans terms or you just don’t want to. If Neils Bohr were alive and you walked into the room, well, he would probably be gentleman enough to ask you to leave maybe telling you you’re in the wrong room or something. He would be right. Thank you for responding metrically also when I asked for mph. Thanks for nothing, better hope you don’t have to go back to teaching. Thanks also for the warning on what I could expect if I opened one o your books. If you have difficulty with English, I’ll be happy to respond back in Espanol, Italian, French or Portuguese although the translations might be a little rough. At this point I don’t see where it would matter. See ya’!

    • William F. Nappi

      btw idiot, (Don) YOU my friend have confused one’s being in need of clarity as being confused in your reference to me. YOU my friend are the one who chose to try to confuse the issue with a lot of “We know what we don’t know..” etc. which out of respect to you I went along with in what I took to be your way of being conversational. My mistake on that one. You’re pretty insecure about your field of endeavor if you ask me.

      Do you even KNOW the difference between Albert Einsteins General as opposed to his Special Theory of Relativity. YOU my friend are POSSIBLY overfunded at FermiLab if that’s the best you can do when someone asks you a simple question that you HAVE to obfuscate the answer.

    • Well it’s sad to lose an admirer, I wish you the best of luck in your search for a clearer science communicator.

      BTW, 220 km/s is 492,000 mph.

    • William F. Nappi

      Yeah, you really did not do a very good job of addressing my questions and when you did, well it was like talking with some company’s customer service rep who’s in India or something, they ask a question, you spell things out clearly then they say something completely irrelevant to what you’ve said. It’s a waste of time. I had anticipated intelligent discussion. Once again I have set the bar too high in expectations. You missed my ‘pointparticle’ implication to another commenter. (Zero-dimensional) It wasn’t my intention to offend. I was hoping for a response. I do NOT appreciate being discredited or told I’m confused my friend. So I have said what I said.

      “A little knowledge confusing me too much???” Really. I was impressed with your credentials but being where you are seems to have led you to some contortionist position where your head is hopelessly crammed up your ass. I was laid up for 23 years friend and I got a lot of research in during that time. For what it’s worth i will leave you with this. You would be wise to give people credit rather than discredit. I will have been on this earth 60 years next year and I have a long academic history myself. You’ve clearly become more successful with your endeavors in the natural sciences, but pissing intelligent people off with military backgrounds and more is a very bad idea.

      Good luck to you.

    • PhysicsLayman

      Dr. Lincoln. How do we know that the amount of non-visible ordinary matter (i.e., planets, gas, etc.) in any given galaxy is not sufficient for maintenance of the galaxy’s structure? If it isn’t, then what percentage of the needed gravity is provided by this matter, on average?

    • Thomas

      Mr Nappi stop taking drugs

    • William F. Nappi

      I don’t touch the stuff (without a prescription.) I DID expect too much apparently in thinking that some type of intelligent, factually based discussion might take place by earlier on stating what I did.

      Instead, to paraphrase in part, I get, “It’s a simple case of limited information…” blah, blah, blah, arrogant head up someone’s butt response.

      Again, my bad. I set the bar too high. Finally, I take a lot of pride in the time I have invested in various educational endeavors, etc. and someone dismissing my comments, feedback, input etc. as ‘simple’ ‘ill-informed’ as a matter of speaking won’t get much respect out of me for long, Thomas. (I’m a complex kind of guy.)

      Your consideration to my sensitivities are acknowledged, appreciated and worthy of respect from afar. Thank you for that. It’s otherwise been quite a day.