The Cosmos


Dark Matter 101

Astronomers have a pretty sweet job. They are paid to stare at the heavens and wonder. Some of their observations are pretty ordinary, but some observations are revolutionary—like the measurements of galaxy rotation that convinced astronomers that our universe is studded with invisible mass called dark matter. In this pencast, I will explain how that apparently simple observation led astronomers to such an extraordinary conclusion.

When astronomers watch rotating galaxies and compare their observations with predictions based on Newton’s laws of gravity, they find something strange. Stars near the center of galaxies are well behaved and move as expected. However stars farther from the center are rebellious. They move far faster than the laws of physics predict they should; so fast, in fact, that these galaxies shouldn’t exist: They should be ripped apart. Since we know that galaxies have existed for billions of years, this is a glaring paradox.

This conundrum nagged at scientists for over half a century. Astronomers proposed many solutions, from suggestions that our understanding of inertia is wrong to new ideas of how gravity works. But the likeliest explanation is that galaxies contain more matter than we see.

When I say “see,” I don’t mean just “seeing” with our eyes or even with the familiar telescopes that are sensitive to visual light. I mean “seeing” with any and every kind of telescope in our arsenal, including the huge antennas that pick up radio emission from the vast clouds of hydrogen that typically make up most of the mass of galaxies.

To acknowledge the fact that this proposed extra matter is invisible to our ordinary methods of detection, we call it “dark matter.” We know it’s out there, but what is it? Come back next week for more about the quest to capture traces of dark matter here on Earth.

Go Deeper
Editor’s picks for further reading

American Museum of Natural History: Vera Rubin and Dark Matter
In this profile, learn how astronomer Vera Rubin’s galaxy observations helped establish the presence of dark matter.

NOVA scienceNOW: The Dark Matter Mystery
In this video, explore the evidence for dark matter.

TED: Patricia Burchat sheds light on dark matter
In this talk, physicist Patricia Burchat explores dark matter and dark energy.

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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 book "The Quantum Frontier" tells the tale of the Large Hadron Collider, the world’s preeminent particle accelerator, while his other book "Understanding the Universe" introduces the armchair scientist to particle physics and cosmology and tells how the two fields are intertwined.

    • Lann_man

      Thank you. That is a really cool way to have a difficult concept explained.

    • Carlos Maurer

      I was surprised by your graph of planet speeds around the Sun, stating that the planets at mid distance travel fastest.

      The speeds of planets I find in and in say simply the the closer the planet is to the Sun, the faster it goes, in contrast to what you state.

      Am I missing something that explains this?

      • Don Lincoln

        You are correct. The closer the sun, the faster the planet. However, the graphic to which you are referrring is not of planets around the Sun, but rather stars, orbiting inside the galaxy.

        What you described applies 100%. However, in the galaxy, the stars near the center “see” the mass inside the orbit, while the stars half way out “see” more mass. Once they get outside the bulk of the galaxy, the mass that an orbiting star sees is just the mass of the galaxy.

        Thus the >>prediction<< says that once you get outside the galaxy, stars way out on the periphery should orbit slower and slower. (Exactly like what you were talking about.) However, they actually don't mover slower and slower. They move at the same speed. This disagreement between prediction and observation is at the core of the dark matter hypothesis.

        Bottom line: Your question is about a solar system, with a small star (compared to the solar system) that contains most of the mass. The graphic that bothered you is about a galaxy, which isn't small and has mass spread out everywhere. This is the fundamental difference between your solar question and the galactic graphic.

        • Michael Polidori

          It would seem that the amount of dark matter necessary to hold galaxies together as one unit would be substantial. Certainly we should have observed some blotting out or magical appearance of light after centuries of observation, but we apparently have not.
          Is dark matter visibly undetectable because it is so small that it doesn’t impede or deflect enough photons to detectably change telescopic images (even through billions of light years of space)?

          • Don Lincoln

            Dark matter does not have electric charge, so light will not interact with it.

            We do see the light deflected by the gravity of dark matter. Look up gravitational lensing to see how this works.

            • Carlos Maurer

              There must exist a mechanism for
              light to interact with matter, even if a particular kind of matter does not
              have an electric charge. This is my reasoning:

              It is well understood that when a
              photon falls on a mass particle possessing electric charge, the momentum
              carried by the photon will transfer to the particle. This happens because the
              electric field of the photon moves the charged particle in the presence of the
              magnetic field of the photon, and this results in a Lorentz force that pushed
              the particle.

              But assume that not one, but two
              photons travel together in parallel paths, and they both simultaneously fall on
              a charged particle. Also assume that the photons are 180 degrees out-of-phase
              with each other, so that they interfere destructively when they fall on the
              particle. The destructive interference would eliminate the electric and
              magnetic fields of both photons, and therefore no Lorentz force could result.
              This means that the momentum carried by both photons would disappear when they
              fall on the particle, violating the principle of conservation of momentum.

              There must exist, in addition to
              the Lorentz force described above, an additional mechanism for light to
              interact with matter, even if a particular kind of matter does not posses an
              electric charge.

            • Michael Polidori

              Are we assuming dark matter doesn’t have an electric charge because light apparently doesn’t interact with it?
              Could dark matter be dwarfed by a photon, which simply envelops it & passes onward without deflection, energy interactions or offering any impediment to the photon on it’s path?

            • Don Lincoln

              This hypothesis seems quite unlikely. Photons are pointlike particles and can interact with electrons and quarks (which are particles that are so small that they have defied any ability to see them). Theoretically, we say the quarks and electrons have no size. Experimentally, we say that they are smaller than 5 x 10^(-20) meters.

              By definition, having an electric charge means being able to interact with photons. So while dark matter remains mysterious and it is OK to think in creative ways, this particular way of thinking is probably not going to be a productive one.

            • Michael Polidori

              But as you already mentioned dark matter and photons do interact. Gravitional lensing. Photons readily interact with many forms of matter (which allows us to see) and their paths can be altered through a glass lense or with a mirror.
              When I say a photon might actually dwarf a particle of dark matter, I said that with full realization of how small photons are guessed to be.
              How small is the smallest bit of matter we have produced through smashing in particle accelerators? My thought is that dark matter particles are many degrees smaller than the smallest in our particle zoo… undetectably small (because of current technological limitations)… other than their inferred presence from dark matter’s gravitional influence… gravity which is deemed necessary to support the stability of galaxies.
              Has there ever been unaccounted for losses of matter or mass in particle accelerators? Could it be we have created undetectable matter through these violent collisions?
              If accelerators can smash particles apart couldn’t we also crush particles into pieces? And then crush the pieces into pieces, until we reached the elementary particle, maximum (not infinite) density?
              Could a supermassive black hole do the crushing?
              In a black hole density increases as space is crushed out of matter. Molecules are crushed into atoms which are crushed into sub-atomic particles, which in turn are crushed into their constituent pieces (quarks, leptons, muons etc) and they in turn crushed… until some maximun density is reached or until the gravitional crushing power of the accumulated matter is reached,
              Infinite density is an abstract thought as even the smallest bit of matter will take up some finite space. Thinking about an infinitely small particle is just as much a waste of time as entertaining infinite density. However, we could play with the idea of MAXIMUM density… which would bring us to the elementary particle… unless the properties of matter are such that, at some stage, a smaller particle has greater volume than the larger one from which it was crumbled…
              Could that be how a black hole or singularity expands or explodes?
              This brings me to my original idea… that dark matter is (are) THE elementary particle(s). I think present dark matter is residual… left over and still not yet combined from either the expansion of the singularity OR the beginning of our universe as a one-of-many-islands in an ocean of THE elementary particle(s). From whence the singularity or ocean of matter come I have not yet thought about… but I imagine an infinite number of universes co0existing in that infinite ocean of dark matter (not parallel or extra-dimensional).
              If the mass of a black hole is sufficiently large I think new dark matter may be created within it… making it a big recycling machine… crushing stardust back into dark matter which (once “released”) combines to form electrons and protons, making hydrogen, which forms stars and then back to stardust… okay!! I think that sews it up for me!!
              Do you have a pin to burst the bubble-universe-black-hole-recycling-dark-matter-mobius-sphere-of-creation-hypothesis?

            • Don Lincoln

              My experience discussing theories generated by interested laymen has taught me that pins that deflate the idea are never accepted. I don’t expect that to be true here either.

              Your idea has the characteristic grand qualities of a layman theory. It incorporates ideas on the nature of dark matter and pulls in black holes. Unifying the cosmic and quantum has eluded a century of professional minds. It is highly improbable that you’ve stumbled on an idea that spans the two.

              However, I have one technical point to consider. I focus exclusively on your claim that photons are simply too big to see dark matter. This is pretty conclusively wrong. As an illustration, consider the wavelength of visible light (about 10^-7 meters). The electron has no known size, but it is smaller than about 10^(-20) meters. Yet the photon interacts easily with the electron. Photons can see small stuff just fine.

            • Michael Polidori

              I am ALWAYS interested in having my bubbles burst… that’s why I invited you to do it. Why would I want to be trapped in ignorance of my own making?
              My aim was not to answer any questions that have “eluded a century of professional minds”, but is a reflection of my effort to make sense out of the information I do have from courses in astronomy physics and chemistry and television shows discussing the latest in cosmological theories & discoveries (the latest stuff fit for general audience consumption and delayed by the time to produce and air the shows).
              The main question I had was about my idea that black holes may be able to crush sub atomic particles into their constituent parts. If that is not possible a simple point to the information that refutes that idea would be appreciated.
              If you do not know, an honest statement to that effect would also be appreciated.
              The massive amounts of dark matter that must exist in the universe (because of the as yet unexplained gravitional force necessary for stable galaxies) and that said dark matter is undetectable led me to the idea that dark matter is undetectable because it may be too small… what could be smaller than particles we can detect… how about the elementary particle?
              If that idea is also ridiculous or unsubstantiable please explain or point me in the proper direction… it puts me further down to the road to understanding if you help.
              I am not put off by being corrected. I welcome it.

            • Don Lincoln

              Well that is a refreshing attitude and one I rarely encounter in venues like this.

              Black holes are able to compress matter considerably. This is just an extension of the neutron star’s ability to compress electrons into the nucleus. But that isn’t such an interesting thing. It’s not like compressing an atom breaks it up…it doesn’t “crush into constituent parts.” Case in point…an atom consists of protons, neutrons and electrons. Those are clear constituents of atoms. Yet a neutron star consists of only neutrons. There is a transformation, not a breaking up into constituents.

              Further, black holes don’t release matter that falls into them. We know that black holes >>cannot<< be the source of dark matter we see. (Meaning compact, high mass, objects are ruled out by the data.) We need a dispersed gas of dark matter to agree with the data. You'd have to figure out somehow to have black holes grab matter, capture it, compress it and then release it (given that not even light can escape).

              To the best of our knowledge, quarks and leptons have no size. Yet photons can see them. While there is some experimental wiggle room, it's hard to imagine that dark matter is too small for photons to resolve. If kilometer-long radio waves can be emitted by electrons (known to be smaller than about 5 x 10^(-20) meters), then gamma rays with a wavelength of 10^(-17) meters can surely see something hypothetically 10^(-40) meters, which is smaller than the Planck Length. Dark matter surely can't be smaller than that.

              This is all very compelling. I don't see any effective counter arguments. If you are unpersuaded, well that's up to you.

            • Michael Polidori

              My idea that sub atomic particles can be crushed into constituent pieces comes from our attempts at smashing particles together in accelerators.
              What is the diffference between smashing particles together at high speed and crushing them against each other with the same (or greater) force?
              Wouldn’t the greater density of a black hole, compared to a neutron star, have to come from the break down of sub atomic particles… in effect squeezing a little more space out of the matter?
              My thought experiment for this treats matter as composed of marbles and each marble is composed of many smaller marbles, and those smaller marbles’ spherical shell containing even smaller marbles.
              In a neutron star, very simplified, we have neutrons packed together with spaces between the touching marble-spheres. Should the gravitional strength increase by addition of matter (or maybe this already occurs in the heart of a neutron star) the neutron marbles begin to break filling up the empty spaces between them with the smaller marbles getting a localized increase in density.
              While the physics of such a transformation is parsecs beyond what I am capable of understanding, does the idea of crushing matter compare in any way to accelerator smashing?
              If smashing particles together can break them apart, I don’t understand why crushing them together with similar force cannot give similar results… the difference being that continuous pressure maintains/contains the constituent particles while increasing density.
              What could the pressures of a black hole or super massive black hole or the singularity do to matter in it’s various transformational stages?

            • Don Lincoln

              Some of what you say is true. However some is misleading.

              A neutron star doesn’t crush atoms into their constituents. You start out with protons, neutrons and electrons and end up with only neutrons. Since the neutron is only one component, the basic premise is falsified.
              Particle accelerators do bring subatomic particles in close proximity. Neutron stars do that as well. Projecting forward to higher densities, so do quark stars (proposed and not observed) and other, denser, forms of matter (also not observed).

              Having particles in close proximity allows for rare interactions.

              The thing that is missing is that the accelerators give particles lots of energy. That energy can be converted into matter via E = mc^2. This doesn’t happen in static crushing.

              And, of course, there is the problem that any crushed matter doesn’t escape a black hole. So your hypothesis is sadly deficient as an explanation of dark matter, which is dispersed and not concentrated in black holes.

    • Michael Polidori

      A collider breaks down known particles by smashing them into one another, breaking particles into bits and pieces of matter that we really don’t understand, the particle zoo
      A thought experiment… a subatomic vise could crush particles, and we have one!! Actually BILLIONS of them.. “ordinary” black holes or super massive ones
      These gravitional vises could crush matter with the same or greater force generated by colliding particles and create the zoo we currently have catalogued within the black holes. An explosion or spewing of matter or energy would release this dark matter into the universe. In the case of energy spewlation, could it “congeal” to form the elmentary particle?

      Inside a black hole my uneducated guess would be the particles are layered according to the force necessary to crush particles into their next lower size and mass
      The true building block of matter, the elementary particle, could explain the properties dark matter has (only indirectly detectable through it’s gravitional influence because of its VSS.
      Black holes, to my indoctored mind, are the logical source of new dark matter, if it is being created now… but I have more thoughts and questions

      If the universe “began” could the big bang have simply created a vast universe filled with dark matter?

      Could the universe have started out as a continguous swirl of dark matter without a big bang?

      Depending on the properties of the truly elementary particle, we would have varying hypotheses & theories about how the particles combine to form other particles, but hydrogen would be the simplest and most abundant element produced… possibly the only element that can be created from dark matter
      Hydrogen, through the creation of stars, si the source of all other atoms, matter, planets, stars and galaxies in our universe…

      The rate at which larger particles & hydrogen formed and the residual dark matter left in any galactic area or “empty” space may give us different clues about the age of the universe and where we are headed.

      Nothing I have found so far or have talked about has countered this explanation of dark matter

      Michael Polidori