Dark matter vs dark energy

Hoosier (below) is a bit confused between Dark Matter and Dark Energy, and unconvinced by the whole shebang. This is very common, so let’s have a post on it..

Dark matter is thought to account for 20% of all the matter/energy of the universe. Although we can’t see it, we’re pretty sure it exists, because its gravitational effect on visible matter can be seen. Put differently, we don’t insist that all existing matter must be ‘visible’ (i.e. emit or reflect electromagnetic radiation). Instead , we include the possibility that some matter may be seen only by its gravitational effect on neighbouring matter. The idea was first postulated by Fritz Zwicky in the 1930s – today, the known motion of certain spiral galaxies suggests that dark matter makes up 22% of all matter/energy, while ordinary (visible) matter makes up only 4% .

Of course, like the MOND crowd suggest, there is always the possibility is that our laws of gravity (both Newtonian and Einsteinian) are simply wrong. But most physicists consider this unlikely, as the predictions of our theory of gravity match observation in so many other instances…

Dark energy is a lot more speculative, and a lot more recent. It’s simply the name we give to whatever is causing the expansion of the universe to speed up (since 1998, it has been known that the expansion rate is currently increasing). The physical cause for dark energy is thought to be some sort of vacuum energy, but nobody’s sure yet. (From the point of view of theory, the phenomenon suggests that Einstein’s equations need an extra term, known technically as the ‘positive cosmological constant’.)

Putting the two together, cosmologists postulate that ordinary matter, dark matter and dark energy all add up to the critical density required for the geometry of the universe to be flat (which is what observation suggests). In other words, the current model of the universe can be summed up by

Density ord matter (4%) + Dens dark matter (22%) + Dens dark energy (74%) = 100%


The strongest evidence yet for dark matter was reported last summer. In the passage of one galaxy through another, one might expect the dark matter of one galaxy to interact differently than its ordinary matter, and researchers at the University of Arizona are pretty sure this is exactly they saw.

Galaxy collision seen by the CHANDRA space telescope

It is also reported here and here that another group, the DAMA-LIBRA collaboration, have observed seperate evidence of dark matter, but this claim is more controversial.


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17 responses to “Dark matter vs dark energy

  1. Thanks so much for taking time out of your day to help explain the mysteries of the universe Doctor C.
    I really do appreciate it!
    a couple more things..That’s a beeautiful picture..but what do the colors of red and blue mean? what does it represent? How does it show dark matter when it can’t be seen? Is that gasses of some sort? how does that prove dark matter?
    Also..I read something somewhere about this guy that put forth the idea that dark energy doesn’t really exist.. the reason it appears to us the the universe is expanding faster and faster is because the speed of light over time is slowing down and makes the red shift skewed to us observers. Have you read that also? Maybe at the big bang light was twice as fast as today..Boy if that was true ole albert would be spinning in his grave! :)
    but if true then e=mc squared would mean that the energy levels near the big bang would have been twice of what we would calculate and slowly everything would be degrading.
    I’ll try to find that link for you to check out.
    Again..thanks for answering my dumb questions..
    I went to one web page and the guy bragged about how smart he was and how stupid i was..I don’t get that alot when speaking in person.. They don’t have the courage to insult you when they know I’d kick thier ass..But this is the beauty of the internet..people can call you stupid and get away with it. Some people are smart..some play sports well..Some know how to fix stuff..why the ego?
    I dunno..thanks again Doc.

  2. cormac

    Re universe expansion – yes, there are alternate theories, including variable speed of light (VSL) theories. They’re v interesting, but they don’t have the support of most cosmologists. I like to concentrate on what 9 out of 10 cats think – on any given topic, there will always be an alternate theory way out there. If the evidence supports it, it will eventually enter the mainstream. Until then, I tend to concentrate on the mainstream (like most teachers!)

    Re image, I haven’t a clue, I’m not astronomer! It’s something to do with the dark matter being less slowed down than ordinary matter, as it encounters another galaxy. (The red/blue stuff is probably an artists’s impression).There are dozens of articles on the discovery in science magazines, if you just search the web under ‘DM and galaxy collision’..Cormac

  3. Bee

    Hi Cormac,

    Thanks for your recent comment. Looks like an interesting blog you have here!

    With apologies for the self-advertisement, we have an introductionary post on Dark matter and one on the Cosmological Constant (meant to write something about Dark Energy more generally, but didn’t quite come around to doing so).



  4. Bee

    btw, regarding the red and blue in the picture. If I recall that correctly, it’s an overlay of two data-sets (and both over the Hubble image with the stars). The red one shows the mass-distribution of visible matter (can’t recall which parts of the spectrum precisely), the blue one shows the mass-distribution as inferred from gravitational lensing data. What you see is that both are offset from each other. The interpretation is that the bulk of the matter which causes the gravitational lensing is not where the visible matter is. The reason why the visible matter is lagging behind is that it interacts more strongly than the dark matter (usually thought to consist of WIMPs – weakly interacting massive particles). Best,


  5. cormac

    Thanks for that Bee!
    Re picture, I suspected as much, but haven’t looked it up in a while.

    Re posts on Backreaction that’s great: I’ll certainly look them up, and refer students to them in future (that said,I suspect your blog may be pitched a level slightly higher than here!)
    Regards, Cormac

    P.S. I see I have my %s wrong…sigh

  6. cormac

    That’s a superb post on the Cosmological Constant, Bee…I’ll certainly refer to it in any post on the subject

  7. Somehow i missed the point. Probably lost in translation :) Anyway … nice blog to visit.

    cheers, Firepower!!

  8. Pingback: Cosmological distance at Trinity College « Antimatter

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  10. RAP

    From my childhood I have always found the concept of “time” confusing – not the number of vibrations of an atom in a crystal but in the larger sense, independent of the environment. For instance what does a picosecond afer the big bang means in an almost infinitely dense media. In terms of dark energy, is it possible that time is a poorly understood scalar that changes magnitude with expansion? For instance as a sphere expands, the volume increase for a shell near the outside boundry is considerably larger than that for an equal thickness near the sphere centre. Is it conceivable that time could be slowing down near the edge of the universe due to expansion (almost like a dilution), or could time be degenerating in the location we consider to be the oldest part of the universe. If matter was travelling at a constant speed (whatever that means) and time was slowing down it would appear that the expansion of the universe was increasing. Further, this concept of time would have an opposite effect at the other end of the spectrum – at the moment of the big bang (in terms of “time” the big bang may have been quite leisurely. This is likely an absurd concept so your insight and comments could help me clear up some fuzzy thinking.



    • earl dillard

      Madam, I fail to see any relationship between disussions of a pure scientific subject on a speculative level with God. If there be any value in your comments it would demonstrate a deep and unrelenting on your behalf, ignorance of bibical teachings. Your fanaticism always reveals a single fact: those of your ilk are ignorant,

  12. Ivanhoe

    Without all the intellectual jargon, simply put, science must conceive of the physical (and anti-physical?) means to explain these things, or rather phenomenon, that are seen, and the unseen causations.

    Basically, while this “faith” in anti-matter, dark matter and dark energy is plausable in the world of science, our unseen Creator is preposterous!

    Isn’t it humourous that our own philosophical babblings of “evidences of the things unseen”, in the physical world, are now used by these brilliant intellectuals to promote their own ramblings as fact?

    “And then an antimatter and matter galaxy rubbed edges, then BANG!”

  13. earl dillard

    As a strict amateur, I have followed the development of the two darks. These sometime outrageous speculations are at times, both scary and downright funny.

    One question I will leave behind for your contemplation: In the formulation of these “theories”, have we considered that accepted laws (Newton, Einstein) do not apply in these unknowns? Perhaps we have difficulty in defining the two “darks” because we are using languages that do not apply.

  14. David Austin

    Funny how social media returns so many comments a second, but the time someone discussed dark matter and dark energy was over a year ago. This site was a top google hit at the time of this posting.

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