Overlay of the profile of the Bullet Cluster measured
using three different techniques. The light orange, round
galaxies that make up the cluster are seen clearly
in the image taken from optical telescopes. Overlaid is the
distribution of gas measured from X-ray observations in red
and the distribution of dark matter in blue. Composite Credit:
X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map:
NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
Optical: NASA/STScI;Magellan/U.Arizona/D.Clowe et al.;

Dark matter continues to be a hot topic at my dinner table. I am continually amazed by how my non-physicist friends are interested in the research on this elusive material that makes up 25 percent of the universe. Just last night I was bombarded with questions like “How do we know it’s out there?” and “Can we see it?”

In my last post, I tried to explain what makes observations of dark matter so difficult. We see ordinary matter through the very strong and efficient electromagnetic force. The problem is that this force does not appear to affect dark matter.

That leaves gravity as the most obvious force for tracking dark matter, but the effects of gravity are really difficult to observe without enormous masses. This difficulty is the main reason that the existence of dark matter wasn’t generally accepted until the 1970’s.

To study dark matter in detail, we have to learn how to pit the forces of gravity against the biggest objects in the universe. One such trick was used in a project at Stanford University, at the Kavli Institute for Particle Astrophysics and Cosmology. In a press release from August of last year, Marusa Bradac describes some pretty convincing observations of dark matter in a massive cluster of galaxies known as the Bullet Cluster.

Measurements of the normal matter in this cluster were pretty easy. The electrons and protons were studied with observations of the hot cluster gas from an X-ray telescope and galaxies from an optical telescope. These can be seen in the figure at the top of the page. Notice the way the X-ray gas (in the hot pink) appears in two vertical bands?

Observations of the dark matter were much more difficult. These researchers measured the effects of dark matter indirectly by weak gravitational lensing. Lensing is a complicated topic that will have to wait for another time, but for the impatient, a couple more good descriptions can be found here and here.

For now, I will just say that we study this hidden matter by its effect on the appearance of the galaxies behind the cluster. The gravity is so strong that it acts like a lens and distorts the image behind it. The blue fog in the figure represents the distribution of dark matter inferred from the distortion of those galaxies.

So why is this interesting? From the image above, we see that the enormous cloud of gas and the enormous cloud of dark matter have completely different experiences in this cluster. The flattened structure of the two bands of gas provides strong evidence in support of a recent collision between two large masses. In the collision, the dark matter passed through relatively unfazed, but the gas in the clusters swirled around like the wind between two fronts in a storm system. In the process, the gas slowed down and is now lagging behind the main material from the two colliding masses. This merger event is demonstrated by a very nice computer animation.

In re-reading this post, I realize that there’s a lot of abstract material. I guess this is why the topic keeps coming up over dinner. Next time I should just apologize to my guests and try to shift the conversation to something less complicated, maybe politics?

Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and
development of a proposed space-based telescope at Lawrence Berkeley National Laboratory

Seeing the Invisible 6 July,2011Kyle S. Dawson

  • mike

    i have a question – it seems like you are now viewing the dark matter by an indirect method. the particles or whatever the dark matter is composed of aren’t radiating energy and light doesn’t bounce off of it. so you are implying that it is there because the light traveling from the stars behind it is distorted by the gravitational field cast by the dark matter. so i guess i am just wondering – if you were there, in that cloud of dark matter in the bullet nebula… then would you see something? could you feel something if you passed your hand through it? is it just invisible because it is so far away? and if not, then what is it made of that allows light to pass through the matter almost unaffected. do you think it is made of particles of some sort such as anti matter particles? but if it were anti-matter light would still bounce off of it, right?

  • Kyle Dawson

    This is a really good question, and worthy of a dedicated post. I hope to write something on direct detection of dark matter in the near future, and will address a lot of these points then.

    Let me say that, yes, all of your points at the beginning are right on. Now, for your questions: if I’m standing in the cloud of dark matter, I still would not see anything or feel anything, because both our sense of vision and our sense of touch are driven by the electromagnetic force.

    How is this??? We see by capturing photons from an object. We use our sense of touch by feeling the pressure of particles against our skin. That pressure is caused by the electromagnetic force. Dark matter does not interact with this force, making it impossible to see or touch, even when it’s right in front of our faces (which it probably is).

    So what is dark matter? We don’t think it is antimatter. Antimatter actually falls into the “normal” category, a weird concept and yet another topic worthy of a future post. We do, however, believe that dark matter is made up of particles, and as I hinted in the last paragraph, we think these particles are passing through me as I write this.

    There are a lot of very complicated reasons we believe that dark matter particles exist. I cannot possible explain these in everyday terms in a 300 word article, much less a 30000 word article.

    Given this impossibility, I will be daring and summarize the reasons in one sentence: we believe another type of particle must be out there to balance the abundance of matter compared to antimatter, and to balance the difference between the way an electric force behaves compared to the magnetic force.

    We have experiments underway to directly detect dark matter particles. They are actually looking for an interaction through a force I have not talked about, the weak force. These particles would then be known as WIMPS, or weakly interacting massive particles. No one has seen anything yet, but I am optimistic that they will in the next 5-10 years.

    I’ve just breezed through a ton of material in this response, if you want to do some further reading, see the following web pages:

    weak force:
    dark matter:

    And keep the questions coming…

  • Where does the neutrino fit into this picture? When I first learned about neutrinoes, they were practically described in the same way that dark matter particles are being characterized today: they only rarely interact with other matter, they’re electrically neutral (although I understand that doesn’t mean they don’t interact with the electromagnetic force)–in effect, they’re all around us, pouring out of the Sun, streaming through our bodies and planet, and yet only rarely would interact with an atom. While I assume we don’t consider neutrinoes as a form of dark matter–they were once characterized as being part of the “undetectable” mass of the universe. Could any of the unseen mass haloes around galactic superclusters, that cause the gravitational lensing that make up the indirect dark matter observations, be neutrinoes?

  • Kyle Dawson

    Neutrinos: I was really tempted to start yapping about neutrinos in this post, for all of the reasons you describe. As always, that presents yet another topic for a large post, but here’s my quick take on the neutrino situation.

    A neutrino does have a lot in common with the proposed dark matter particle. It does not interact with the electromagnetic force, they’re all around us, they rarely interact with other particles, etc. There is one distinct difference, there are a LOT of neutrinos. Neutrinos are actually a bi-product of quite a few quantum mechanical processes. A lot of radioactive decay produces neutrinos, and the sun produces neutrinos as it burns hydrogen into helium.

    It was through observations of these processes that people first realized the neutrino must exist. They found that they could not account for all of the energy during radioactive decay, some energy seemed to just disappear. This violates the law that energy must be conserved, so it quickly became obvious that the original theory needed to be updated to include a new particle. Fortunately, the sun creates a lot of these unseen particles, so we can now almost routinely detect them, now that we know what we’re looking for.

    We’re not so lucky with dark matter. We don’t believe dark matter particles are routinely created or destroyed, and we don’t believe there are as many dark matter particles as there are other more familiar particles. This may be counter-intuitive, since I described before that dark matter makes up a larger fraction of the universe than does normal matter. The difference is the mass, one dark matter particle is suspected to be 100s of times more massive than a single proton. The relatively small number of particles makes dark matter much harder to detect than the extremely common neutrino.

    We actually have a pretty good understanding of neutrinos, but a lot of work needs to be done. We have estimates of the number of neutrinos in the universe, and we know they have a mass, but we don’t know what that mass is. It is extremely small, too small to be a major component of unseen mass haloes around clusters or galaxies.

  • mike

    that’s interesting – so how are neutrinos directly detected? they are more than just theory, right?

  • mike

    i posted too fast – just found my own answer:


    “In this experiment, now known as the neutrino experiment, neutrinos created in a nuclear reactor by beta decay were shot into protons producing neutrons and positrons both of which could be detected.”

    Are the experiments to detect dark matter similar? What i mean is this: For neutrinos it was theorized that they were present in radioactive decay and nuclear reactions. those are both events that we can stage on earth in a laboratory. So scientists were able to run an experiment that would in theory create neutrinos, and so they just had to figure out how to detect them once they were created. that was done by smashing them into protons.

    so for dark matter do we have an idea as to how to create it in a laboratory environment? and if created could we detect it by smashing it into something else or maybe by watching how it affects a gravitational field (or how it affects the weak force like you mention above)? or does the experiment instead rely on the constant stream of dark matter rushing around everywhere all the time?

  • Kyle Dawson

    Neutrinos have been detected from several sources, one of which is nuclear reactors. We have also directly detected neutrinos from the sun and even from a supernova that went of in the Large Magellanic Cloud:


    Right now, the direct searches for dark matter are more similar to experiments that look for neutrinos from astrophysical sources. The evidence strongly suggests that there is a population of dark matter particles in the galaxy, and all dark matter experiments are looking for these particles.

    In the very near future, the particle accelerator at CERN hopes to achieve energies high enough to create dark matter particles in the laboratory environment. If it does, we won’t actually detect the dark matter particles directly, but rather by the same methods which the neutrino was originally discovered, by finding massive amounts of energy unaccounted for after particle collisions. There will not be enough particles created to detect them directly, the interaction is just way too weak, and gravity of individual particles is practically non-existent.

  • Kyle Dawson

    response to:
    i’m curious about the dark matter detection – specifically what is the deal with the weak force that you mention? are the experiments trying to stage beta decay and somehow funnel dark matter into that event to see if it interacts?

    Yeah, the weak force is actually really hard to understand intuitively, we don’t see it on macroscopic levels the same way we see gravity and E&M, and it’s not as obvious as the strong force which can hold together the positively charged quarks in a proton. Mostly, it is seen in forms of radioactive decay when a proton turns into a neutron, or when a neutron turns into a proton. These are beta decays, which are different from alpha decay where a helium nuclei is emitted. It is fairly weak, which is why some unstable elements can take a very long time to decay, (the half life of tritium is pretty damn long).

    The experiments to detect dark matter aren’t actually trying to stage beta decay, they’re looking for a weak interaction more similar to the E&M we are more familiar with. Here, a cosmic dark matter particle comes flying through the detector, and puts a small force on a nucleus in the Germanium crystal. The nucleus moves, and causes a wave throughout the entire lattice which can be measured directly. It’s like hitting the lattice with a hammer. Of course, a lot of things can hit the lattice like a hammer, which is why the experiments are in the deepest mines to provide some shielding from all the stuff that’s flying around the atmosphere.


Kyle S. Dawson

Kyle Dawson is engaged in post-doctorate studies of distant supernovae and development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.

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