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CERN | Geneva | Switzerland

View Blog | Read Bio

Cosmology and dark matter

Third part in a series of four on Dark Matter

I have already reviewed how it reveals its presence through gravitational effects and the lack of direct evidence of interaction with regular matter. Let’s now look how cosmological evidence also supports the existence of dark matter.

Galaxy seeds

It is now widely accepted that all matter (dark and visible) started out being uniformly distributed just after the Big Bang. To make a long story short, a rapid expansion followed where the Universe cooled down and particles slowed down enough to form nuclei three minutes after the Big Bang. The first atoms appeared 300 000 years later while galaxies formed between a hundred and a thousand million years later.


How did the Universe change from being a gigantic cloud of uniformly distributed matter to containing large structures?  Dark matter is probably the one to be blamed.

Dark matter is heavier than regular matter and slowed down earlier. Small quantum fluctuations eventually turned into small lumps of dark matter. These lumps attracted more dark matter under the effect of the gravitational attraction, in a very slow snowball effect. Since dark matter also interacts very weakly, these planted seeds survived well through the stormy moments of the early Universe.

Once matter cooled off as the Universe expanded, it started accumulating on the lumps of dark matter. Hence, dark matter planted the seeds for galaxies. “All this could have happened without dark matter, although it would have taken much more time,” explains Alexandre Arbey, theorist at CERN.

Simulating the formation of the Universe

Not convinced? Nowadays, scientists can reproduce this process using computer simulations. As a starting point, they inject into their models how much matter and dark matter there was right after the Big Bang. The observations of the cosmic microwave background provide these estimates. Then they let it evolve under the attractive effect of gravity and the repulsive effect of the Universe expansion.

All these guesses must converge to reproduce the amount of dark matter leftover today, a quantity called the “relic abundance”. If all is properly tuned, scientists can recreate the whole evolution of the Universe in fast motion from the moment of the Big Bang until today.computer-simulation

The results are striking as can be seen on the three pictures above. These computer-generated images show the distribution of dark matter 470 million years after the Big Bang, then 2.1 and 13.4 billion years later (today). Dark matter first formed small lumps, then long filaments and finally large-scale structures appeared.

Scientists from the French National Centre for Scientific Research (CNRS) just released an amazing video showing how they are now using these mega simulations in the hope to discriminate against different dark matter and dark energy models by comparing these images with current observations.

Cold dark matter

Another way to figure out which theory of dark matter best fits the reality was provided last month by a group of scientists working with the Subaru telescope. They studied the distribution of dark matter in fifty galaxy clusters. Averaging all the data, they found that the dark matter density gradually decreases from the centre of the clusters to their diffuse outskirts.

This new evidence conforms to the predictions of cold dark matter theory (CDM), which states that dark matter is made of slow moving particles. Hot dark matter candidates like neutrinos would be made of particles moving close to the speed of light.


Cold dark matter theory predicts that central regions of galaxy clusters have a lower dark matter density while individual galaxies have a higher concentration parameter.

Unexplained signals from outer space

Astronomers are not just providing clues to the mystery of dark matter but also raising questions.  For example, a decade ago, the INTEGRAL-SPI experiment found an intense gamma ray source at 511 keV coming from the galactic centre, exactly where dark matter is most concentrated. This value of 511 keV is precisely the energy corresponding to the electron or positron mass.


This smelled incredibly like dark matter particles annihilating or decaying into pairs of electron and positron, which in turn can annihilate into gamma rays as depicted on the diagrams above. Unfortunately, nowadays the excitement has somewhat wound down since theorists have a hard time reconciling its characteristics with numerous other observations.

Several satellite experiments (HEAT, PAMELA and FERMI) have observed an excess of positrons in cosmic rays. A positron is the antimatter counterpart of the electron. Given matter prevails over antimatter in the Universe (otherwise, we and the galaxies would not be there), astrophysicists have to figure out where these positrons come from.

Many theorists have attempted to explain this in terms of astronomical phenomena but the jury is still out. Could this be the first concrete sign of dark matter? The AMS experiment on-board the International Space Station has already shown that they have high quality data and could provide a definitive answer very soon.

Dark matter remains a mystery but this field is fast evolving. In my next blog, I will look at what the Large Hadron Collider (LHC) at CERN could do after restart in 2015.

First part in a Dark Matter series:       How do we know Dark Matter exists?

Second part in a Dark Matter series:  Getting our hands on dark matter

Third part in a Dark Matter series:     Cosmology and dark matter

Fourth part in a Dark Matter series:  Can the LHC solve the Dark Matter mystery?

Pauline Gagnon

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  • James Johnson

    Dear Pauline,
    If both dark and regular matter feel the gravitational force, then black holes must be composed of both. Given the lumpy distribution of the early universe, black holes must also show a distribution of matter/dark matter.
    Would that not leave an observable trace? E.g., particle pairs created at the event horizon, one escapes and is observable, one disappears…would that not also be true of a dark matter particle?
    james johnson

  • toy


  • Botond Barabas


    could black holes be “made” of dark matter?
    As you wrote dark matter is heavier than visible so it has a stronger gravitational force.
    It could be that the black holes are actually a super small ball of super concentrated dark matter.

    What do you think?


  • Goutam Barik

    It has helped me to understand the basic idea of dark matter in very simplest way.

  • http://www.cern.ch CERN


    good try but black holes are made of regular, visible matter. It is created from massive stars that collapse upon themselves under the effect of their own gravitational attraction. You can read more on wikipedia on this: http://en.wikipedia.org/wiki/Black_hole#Formation_and_evolution

    I hope this helps, Pauline

  • ian

    Has anyone fired a beam of positrons, probably from an unstable isotope, at an insulated metal? If so what happened, or, would be expected to happen?

  • John

    What if…

    What if the cosmological observations that cause us to devise particle theory solutions aren’t due to particles at all?

    Let me explain…

    It is clear that most current approaches to explaining dark energy and matter are extrapolations of current theories… no surprise, that’s how physics typically works. We leap to devising new particles that must somehow interact in a way that results in our observations. What if this approach is flawed?

    What if there is some other fundamental symmetry or degree of freedom in nature that is not apparent at our observation scales that manifests itself in the phenomenology we observe? Our understanding of the quantum realm was a similar leap. Our physics is limited by our constrained formalism just as classical formalism was inadequate to describe quantum behavior. Though once the proper quantum formalism was understood classical physics could be derived.

    Barring an experimental breakthrough that leads to a theoretical enlightenment, theorists need to look beyond current marginal extensions in formalism to explain the observed.

    Be Bold!

  • Glenn

    Presumably dark matter would fall into black holes though wouldn’t it? and then there seems no reason why there couldn’t be a black hole formed from dark matter.

  • pradeesh

    sir, i think about black matter
    the basic form of any matter
    that is electron and protons, neutrons not sustained to any matter in the universe
    basic energy = electron + protons + neutrons (and both of them)
    pradeesh p
    austin town
    bangalore india
    phone No +919249335566

  • http://www.cern.ch CERN


    you are right: the basic form of any matter ***we know*** is made of protons, neutrons and electrons. To be more general, one could say quarks and leptons. But that is exactly the problem: I am talking here about a new and completely different type of matter not made of quarks and leptons, which is why it is so puzzling. Moreover, we have not yet figured if and how it interacts with ordinary matter (quarks and leptons) except through gravitational effects.

    And by the way, I am not a sir. There are many women physicists so do not make any assumption even when you cannot tell by the name.

    Cheers, Pauline

  • http://www.cern.ch CERN

    Indeed, Glenn, dark matter would feel the gravitational attraction of a black hole. And yes, there might also be black holes made of dark matter.

    Cheers, Pauline

  • http://www.cern.ch CERN

    Hello Ian,

    this has nothing to do with the topic, unless you think dark matter and antimatter are the same, which they are not. So just to be sure you are clear on this, antimatter is part of what we call ordinary or visible matter. It is made of anti-quarks and anti-leptons, but that is still regular matter.

    As you know, positrons are the antimatter counterpart to electrons. When the two meet, they annihilate in the form of two photons and leave off energy. If you shoot any type of matter with a beam of positrons, be it a conductor or an insulator, every positron that will meet an electron will annihilate and give off energy. In fact, what happens in both cases, the positron will form a short-lived bound state with an electron called positronium – that’s just like the hydrogen atom but with a positron instead of a proton. This lives between picoseconds and nanoseconds before annihilating into 2 photons. It leaves longer (about 140 nanoseconds) if the positron and electron have their spins in the same direction (you get a state called ortho-positronium). It disappears really fast (like 100 thousand times faster, just a few picosecond) if they have opposite spin (you get para-positronium). The irradiated material will charge up since you are taking away some of its electrons. For your insulated conductor, the metal will charge up whereas the insulator will accumulate charges then create a spark (a discharge) to the metal.

    I hope this answers your question. Pauline

  • ian

    Is the amount of dark matter in the galaxy increasing, decreasing, or staying the same?

  • http://www.cern.ch CERN

    Good question. My cosmologist friend tells me it’s about constant. She says: roughly, the total number of dark particles in the universe has remained constant since the `freeze-out’ which happened a long time ago, when the universe was some 10^-9 second old (nanoseconds), well before the light from the Cosmic Microwave Background (CMB) was emitted. The Planck satellite results using data from the CMB and gravitational lensing give us the amount of dark matter today. Even if dark matter annihilates into regular matter, it would only occur in the denser areas and can be neglected.

    I hope this helps, Pauline

  • http://www.cern.ch CERN

    Perfect, I am glad to hear that! Cheers, Pauline

  • http://www.cern.ch CERN

    Good try! I agree with you that black holes must contain some dark matter. And that given the strong gravitational field, the concentration of dark matter will also be larger than around the black hole. But if the massive star that collapsed in the first place to form the black hole was far from the galaxy center, the availability of dark matter would not be there and it would not work.Also, what you propose for detection relies a bit on luck: the annihilation must occur just at the black hole horizon, and sufficiently often that a telescope or a detector on Earth would catch that signal. Sounds tough to me.

    What I have seen so far is that people (i.e. the experts in the field) tend to look in the center of galaxies to find sources of dark matter annihilation since this is where it is most concentrated. So my guess is that this is indeed the best place to look.

    Cheers, Pauline

    I think it is my guess against yours… Cheers, Pauline

  • ian

    Hi Pauline
    Don’t know if this is the right place, but what’s your take on higgs boson particle(s) and dark matter

  • Amir Livne Bar-on

    What about in the other direction? Is dark matter expected to come out of a black hole as Hawking radiation? If so, can this be one reason this radiation was not detected?

  • http://www.cern.ch CERN

    Sorry, I do not know the exact answer here. I am not an astrophysicist, just a particle physicist. I will need to find a colleague to help me on this but I am about to leave for a conference, so bear with me. I will answer you as soon as possible.


  • Amir Livne Bar-on

    Sure, no rush :)

    You’re answering an impressive number of questions anyway, I for one am learning a lot from this series and really enjoying it.

  • Laurence Cox

    Pauline, while you are quite correct about stellar-mass black holes, I think that the origin of super-massive black holes is still an open question. Their link to quasars shows that they developed in the very early Universe, so something that slowed down earlier than normal matter may well have formed the seeds of the supermassive black holes that are now found at the centres of galaxies. You don’t actually need a high density for a super-massive black hole to form.

  • http://www.cern.ch CERN

    Thanks for adding more info here. Cheers, Pauline

  • http://www.cern.ch CERN

    Good comment. Indeed, only experimental proof will determine what is real in the end. And you are right, this is a very complex problem and we ought to be bold to tackle it. It is of course simpler to imagine things similar to what we know for regular matter but there is no warrantee it will be the case.


  • http://www.cern.ch CERN

    Sorry, I had missed your comment.

    I do not have a particular preference, even though I worked for years designing a way to see if Higgs bosons decayed to dark matter particles. I mentioned this work in my last blog in this series. There are current searches for that both in CMS and ATLAS and we should see the results in the coming months. To the least it will limit what can be there.

    One thing is clear: we must leave no stones unturned. Looking for Higgs bosons decaying into dark matter is one of them.


  • Varun

    I’m kinda in a fight with my science teacher and I really love physics so I’m a big fan of your answering Pauline so I just wanted to know if conduction can take place in a black hole. I know it’s a simple question but just wanted to make sure.

  • http://www.cern.ch CERN

    Hello Varun,

    I would not call that a simple question… Are you talking about electrical conduction? If so, I doubt it since matter gets compressed in a black hole so I imagine the valence electrons collapse onto the nucleus, just like the other electrons. So conduction would not happen… Just my own guess here.

    Cheers, Pauline

  • caw

    “black holes are made of regular, visible matter” ??? Black holes may be formed from visible matter but not in any sense do they contain regular, visible matter.

  • caw

    Hawking radiation at the boundary of a black hole does not depend upon the form of the contents of the black hole – only what happens outside the event horizon.

  • caw

    Dark matter, if it exists, is a form of matter lower in energy to “regular visible” matter. Dark matter probably won’t decay into regular matter but perhaps the other way around – with regular matter decaying towards a lower energy state – dark matter.

  • http://www.cern.ch CERN

    Indeed, you are right. A black hole is made of regular matter that collapsed on itself such that the distance from the nucleus to the electrons is reduced to very little. In that sense, it has little in common with regular matter. My point was just to stress that initially, a black hole is made of regular, visible matter.

    We both agree on that. Thanks for clsrifying. Pauline

  • caw


    What’s your opinion on cases made for dark matter stemming from quantum fluctuations of virtual particles? If the matter/anti-matter pairs both have positive mass, they might exhibit momentary gravitational fields. If the pairs somehow exhibit +/- gravitational properties, they might still become gravitationally polarized by existing normal matter, thus creating “dark matter” in the vicinity. Any thoughts?

  • http://www.cern.ch CERN

    Interesting idea, I had never heard of this before, which means I will not be able to comment. Two experiments at CERN wil soon attempt to verify if antimatter (namely anti-hydrogen atoms) behave like matter in the presence of the gravitational field. More specifically, they will attempt to measure the gravitational constant g for antimatter. Is g the same as for us? The expected answer is yes but it remains to be tested. Indeed, the mass is positive for both matter and anti-matter but I have no clue if such quantum fluctuations as what you describe is possible or not. To be watched.

    Thanks for sharing these thoughts, Pauline

  • caw


    Thanks for your reply to my question on quantum fluctuations.

    I realize that it is very unlikely that anti-matter would exhibit anti-gravity, but the g forces involved may not be identical.

    Here are a couple of articles related to my first question:



    Then gravitationally polarized matter/anti-matter (where anti-matter exhibited anti-gravity) would also isolate anti-matter away from normal matter and might thereby contribute to the expansion of the universe:


    Thanks again,

    Charles wright

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  • Gualdo

    Dear Pauline, many thanks for this great blog.
    I understand that dark matter associated with a given Galaxy is less concentrated than ordinary matter within it, as otherwise it would not be possible to account for the rotation curve, correct? Then we have dark matter “halos” around visible matter in galaxies; is all this correct?
    However, if dark matter first concentrated into lumps short after the Big Bang, and much later visible matter started to concentrate around those lumps, wouldn’t one expect the opposite, ie that dark matter would be more spatially concentrated than ordinary matter? What is the the silly thing I’m missing in this argument?