Dark Matter

Posted on by Steve Hurley

This post is about dark matter and is the latest in my series on cosmology, the study of the origin and evolution of the Universe as a whole. As readers of my previous posts will recall, dark matter makes up about 27% per cent of the mass of the Universe.

Dark energy now

Evidence for dark matter

Our solar system contains the Sun, eight planets with their moons and various minor bodies such as dwarf planets, comets and asteroids. If we plot the speed that each planet orbits the Sun against its distance from the Sun, then we get the curve shown below.

Planets speed

The graph above shows the speed at which the planets orbit the Sun in kilometres per second, plotted against their distance from the Sun in astronomical units (AUs). 1 AU is just under 150 million km and is the average distance between the Earth and the Sun.

The way that the speed of the planets’ orbits falls off with their distance from the Sun indicates that nearly all the mass of the Solar System is concentrated in its centre at the Sun. The further away a planet is from the Sun, the weaker the Sun’s gravitational pull and the more slowly it orbits. See Note 1.

Jupiter, the most massive planet, has only 0.1% the mass of the Sun. In fact the total sum of the masses of all the planets, their moons, dwarf planets (like Pluto), asteroids and comets in the Solar System is less than 1% of the mass of the Sun. This means that the effects of gravity caused by the other bodies in the Solar System on the speed of the planets’ orbit are insignificant.

The Sun belongs to the Milky Way galaxy, which contains about 400 billion stars (Cain 2013).  If you were to look at the Milky Way from a great distance, it would look as shown below.

Sun in Milky Way

What the Milky way galaxy would look like from outside

All the stars in the Milky Way rotate around its centre, and the Sun rotates at a speed of 782,000 km/hour (Cain 2008).  However, the distance between the Sun and the galactic centre is so great (nearly 30,000 light years)  that it takes around 230 million years to complete a full revolution. This vast period of time is sometimes called a cosmic year.

Most of the stars in the Milky Way are concentrated near its centre.  So if, like the Solar System, most of the matter in the Milky Way were in the form of stars, then it too would be concentrated at its centre. We would expect that as we get further from the centre of mass, then the stars would revolve around the centre of the galaxy more slowly in the same way that the planets orbit more slowly as we get further from the Sun.  We would expect a rotation curve (a plot of the speed that a star orbits the centre of the galaxy against its distance) in which the orbital speed falls off with distance from the galactic centre, similar to A in the diagram below.

Galaxy Rotation Speed

In fact the orbital speed of a star around the centre of the galaxy does not fall off with its distance from the galactic centre. The rotation curve for our Milky Way galaxy is actually like B in the diagram above. The only way that these results can reconciled with our existing laws of physics is for there to be a large amount of matter in the outer regions of our galaxy which is not in the form of stars. The pull of gravity due to this matter means that the rotation curve does not fall off with distance. Because it does not emit light it is called dark matter and, to produce the flat rotation curves observed for our galaxy, most of its matter must be in the form of dark matter.

In the 1970’s astronomers measured the rotation curves of other spiral galaxies.  It became clear that all spiral galaxies had rotation curves in which the speed at which a star orbits the centre of the galaxy does not decrease as a function of the distance from the centre of the galaxy.  An early pioneer of this work was the American astronomer Vera Rubin (1928- ) pictured below.

Vera Rubin

In an influential scientific paper presented in 1980 she and her colleagues presented observations of the rotation curves of a large number of spiral galaxies (Rubin et al 1980). All of these showed rotation curves similar to the Milky Way. To explain her observations, spiral galaxies would need to be surrounded by an invisible dark matter halo which would, in general, have about five times the amount of matter that is held in the galaxy in the form of stars.

dark matter halo

This diagram shows a typical spiral galaxy surrounded by an invisible dark matter halo. The bright centre of the galaxy is shown in white and the outer regions of the galaxy are shown in light brown. The dark matter halo, which although shown in blue is invisible, is not flattened in a disk like the galaxy and extends to about 3 times the galaxy’s radius.

Since Rubin’s pioneering work, it is now generally accepted that most of the mass of galaxies is in the form of dark matter.

Clusters and groups of galaxies.

In general, galaxies are found in groups and clusters, the largest of which contain thousands of galaxies. The speed at which these galaxies are moving with respect to each other in these groups and clusters is often very high. For large clusters, such as the one shown below, individual galaxies can be moving at speeds of over 1000 km/s (3,600,000 km/h) relative to each other.   To prevent the galaxy groups and clusters from flying apart, something must be holding them together. The most widely accepted explanation of this is that there must be a great deal of dark matter in most galaxy groups and clusters, and it is the force of gravity due to all this dark matter which binds the cluster together.

virgo-galaxies-cluster

Part of the Virgo Cluster, a large cluster of galaxies about 50 million light years from Earth

Other Evidence for dark matter

Other evidence for dark matter comes from gravitational lensing, where the strong gravity from clumps of dark matter which lie between a very distant object and Earth actually form a “gravitational lens” and bend light rays causing two images of a very distant object to be seen.

gravitational-lensing

A gravitational lens caused by the large amount of dark matter around a cluster of galaxies (D) causes two separate images (B and C) of a very distant galaxy (A) to be seen.

I won’t say any more about this in this post, but if you would like to know more about gravitational lensing caused by dark matter click here for an interesting article from the phys.org website.

In addition, cosmologist believe that clumps of dark matter were the seeds of galaxy formation. Without dark matter there wouldn’t be enough matter for galaxies to form. How galaxies form is such a huge topic that I could write several posts about it so I will come back to this at a later date.

What is the nature of dark matter?

As you will recall from my previous post ordinary matter is made up of atoms. Some dark matter may be in the form of ordinary matter in objects such as brown dwarfs. These are objects which are midway in size between the lightest stars and large planets such as Jupiter. Because they emit little or no light, brown dwarfs are extremely difficult to detect.

Brown dwarfs

However, for reasons which I’ll discuss in a future post, cosmologists believe that although some dark matter is in objects such as brown dwarfs, most dark matter isn’t made up of atoms. Instead it is made up of an entirely different kind of matter altogether. Various candidates have been suggested for the particles which make up dark matter but none has ever been detected by astronomers or in any particle physics experiment.  The nature of dark matter is one of the great unsolved problems in physics.

Related Posts

This post is the fifth in my series about cosmology. The other posts are:

(1) The Universe Past, Present and Future. This describes what is meant by the Universe and gives an overview of its origins, evidence for its expansion and discusses its ultimate fate. To view this post click here.

(2) A brief history of the Universe.  This gives a history of the Universe from just after the big bang until the current date. To view this post click here.

(3) Dark Energy. This post gives the reasons why cosmologist believe dark energy exists and why it makes up nearly 70% of the mass of the Universe. To view this post click here.

(4) Dark Energy over Time. This post discusses how the amount of dark energy in the Universe has varied over time and its implications on its future evolution. To view this post click here.

Notes

1 It is fairly easy to show, using high school physics, that if all the matter in the Solar System is concentrated in the Sun, then the speed of a planet’s orbit is proportional to the inverse square root of its distance from the Sun.

References

Cain, F (2008) Sun orbit, Available at: http://www.universetoday.com/18028/sun-orbit/(Accessed: 19 February 2015).

Cain, F (2013) How Many Stars are There in the Universe?, Available at:http://www.universetoday.com/102630/how-many-stars-are-there-in-the-universe/(Accessed: 19 February 2015).

Rubin, V. C.; Ford, W. K. & Thonnard, N. (1980) Rotational properties of 21 Sc galaxies with a large range of luminosities and radii, from NGC 4605 (R = 4kpc) to UGC 2885 (R = 122 kpc). The Astrophysical Journal, Vol. 238, 471–487

Published by Steve Hurley

Hi I am Steve Hurley. I work in the IT industry. I studied for a PhD in astronomy in the 1980s. Outside work my real passion is explaining scientific concepts to a non-scientific audience. My blog (explainingscience.org) covers various scientific topics, but primarily astronomy. It is written in a style that it is easily understandable to the non scientist. Publications and videos For links to my books and videos please visit www.explainingscience.org

46 thoughts on “Dark Matter”

  6. […] 2. Most cosmologists now believe that 85% of the matter in the Universe is another mysterious form of matter called dark matter. No one knows what dark matter consists of, but is clear is that it not made up of atoms in the same way as ordinary matter. Dark matter cannot form stars and does not clump together to form structures like gas clouds in the same way that ordinary matter does. It is completely invisible to telescopes because it is transparent to light. However its existence is inferred because of its gravitational effects on visible matter. Dark matter is covered in my post https://explainingscience.org/2015/02/18/dark-matter/ […]

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  12. Nice informative article on the current consensus, why dark matter is assumed/predicted/required if SCM-LCDM to exist.
    ‘The only way that these results can reconciled with our existing laws of physics is for there to be a large amount of matter in the outer regions of our galaxy which is not in the form of stars. ‘
    From my perspective it should be qualified as ‘one way these results can be reconciled’ or ‘currently, the only ..
    as the same factors that come into play in ‘SPIRAL cosmological redshift illusion resolution’s ‘Black-hole illusion resolution can explain the galactic rotation issue being due to hyper-dense proto-galactic formation, then a rapid expansion epoch to mature density, relatively early in the history of the universe. .

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  16. You’ve given a comprehensive explanation about dark matter. This is a wonderful post indeed. Your references are especially good. Looking forward to more such interesting posts 😃👌

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  17. Firstly, thank you for liking my own post on my blog, and secondly, you have a new follower! This was certainly a lovely read! And with an interest is astrology and space in general myself I look forward to reading more of your work! Extremely refreshing and informative. Thank you.

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  19. Reblogged this on the naked human and commented:
    it seems that matter itself is 99.999999% empty space, and even that is only 5% of “Reality”, so more elusive even than almost empty matter is so called dark matter and more elusive than that is dark energy, could it be that the whole universe consists of thought, human or otherwise?

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  20. Reblogged this on Chaos Theory and Human Pharmacology and commented:
    Sometimes I think that we understand drugs as much as we think we know the space just by seen the stars from the balcony of an apartment in the central park of New York at 8:00 PM (on a summer day).

    There are many unpublished clinical trials that we don’t not know nothing except for their registry on a public database (e.g., ClinicalTrials.gov). That’s something like the stars we cannot see
    from the balcony, but we know that these stars are there, probably we can see that stars if we were outside of the city.

    Now, let’s change the subject, what about clinical trials which are unpublished and unregistered? That’s probably something that we understand even less than the astronomers doing research on the dark matter. There is evidence of that somewhere?

    Oh yes, agomelatine: http://www.bmj.com/content/348/bmj.g1888/rr/762419

    More evidence is out there, the problem is that databases of drug studies are huge, and not enough people doing research on the frauds of medical research.

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  21. Could Dark Energy,matter and flow be a form of condensed liquid? Super heated fuidic form cobtrolled by an electric form or energy?

    P:S I have an affinity towards a consciouness controlling this electric energy.

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    1. Hi thanks for your comment

      To answers your question I think it is very unlikely 😉
      I am more inclined to go along with the more accepted theories of dark matter and dark energy as outlined in my recent posts.
      The Science Geek

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      1. arjundass, dark matter has no electromagnetic interactions. That is what makes it dark (it neither emits nor absorbs electromagnetic radiation.) Also, computer simulations show that it behaves according to the laws of gravitation. There is nothing directing its behaviour.

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  22. Thank you for visiting Envisioning Life. I am glad you found something of interest that you could enjoy. Thank you again.

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  24. Very interesting and informative posts.

    “To explain her observations, spiral galaxies would need to be surrounded by an invisible dark matter halo which would, in general, have about five times the amount of matter that is held in the galaxy in the form of stars.”

    From a post about Dark Energy we learn about Dark Matter that, “most cosmologists believe [it] has a density of roughly 0.0027 trillionths of a gramme per cubic kilometer of space when averaged out throughout the Universe”

    One assumes then that DM is greatest around galaxies, as per your illustration above. I picture something fluid, almost like water, transparent, yet still very there. Besides gravitational lensing, I wonder what other effects such mass may have on our observations, what other possible distortions, blocking etc? On earth, due to water vapor in the air, mountains at distance appear blued.

    Anyway, just some thoughts. Again, interesting posts.

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    1. There’s a couple of things here that bear questioning.
      First, Keppler’s third law- this is something that we know to be true in our solar system, and we see it in application with all the planets and with our sun… does it always work everywhere? Are there places where the 3rd Law cannot be proven to be operational? That is, that we’re fixated on “this law works always everywhere, except here.”

      Thus, the conclusion that dark matter exists comes about because we cannot question Kepler’s 3rd law, or perhaps we’re just wrong on that one. Works with solar systems, but galaxies? Isn’t the law based on a newtonian constant, which works with the gravity which is determined by mass, right?

      It just seems odd that we have ye olde dark matter out there, when it’s not substantiated by any physical evidence, and you cannot reproduce it, but you have to assume it’s there based on the precision movements of the galaxies.

      And where does the dark matter inhabit? Just the outer reaches of the solar system? Why doesn’t it exist in a solar system like ours? Shouldn’t the dark matter give off some sort of spectrographic effect?

      It is, or should be, the most plentiful stuff in the universe yet we cannot detect it, see it, or even determine what it is. Except for the math problem of the galaxies not behaving the way Kepler thought they should, I think we might have come up with something that cannot be verified through experimentation, is not visible to the eye or any spectrum with which we work, and, oh yeah, it’s a majority of stuff in the universe, just that we can’t figure out what it is.

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      1. Thank you pontiuscominius and midmiocene for your interesting comments.
        As discussed in my post and some of my follow-up comments the current view of most cosmologists is that dark matter is composed of an unknown form of matter which has never been observed directly because it doesn’t interact (i.e. appears transparent to) with light on any other form of electromagnetic radiation. It can only be inferred because of its effects on ordinary matter.

        A small minority of cosmologist believe that one way around this problem is that Newton Laws of gravity don’t hold at a very weak accelerations. This alternative theory is called Modified Newtonian Dynamics (usually abbreviated to MOND) and was first proposed by the Israeli physicist Mordehai Milgrom in 1981. More details are given in the link below

        http://physics.about.com/od/physicsmtop/g/mond.htm

        The Science Geek

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      2. Hmm. Something that gives all the appearances of being there but which is undetectable. Of course i’m just tossing out ideas, and obviously water couldn’t be in a liquid state in space. Then, when I look at the drag-like arms of the galaxies that gives the appearance of differential speed. What if they at one point were moving slower, but then sped up again for some unknown reason? All wild SF nonsense of course, but I thank the Science Geek for indulging me.

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      3. pontiuscominius, your thoughts were my initial thoughts on first reading about dark matter some years back. But there is more evidence than just rotations of galaxies. Gravitational lensing has been mentioned in this article. Also collisions between galaxies provide evidence for a halo of dark matter in and around all galaxies. Modified Newtonian dynamics can’t match the empirical results.

        The dark matter fills the galaxy and an area of space around it, and is present throughout the solar system. However, it’s density is low and as it does not interact via the electromagnetic or nuclear force, it is extraordinarily difficult to detect.

        Unlike normal matter, it doesn’t clump together to form stars or planets. It is instead dispersed throughout space like a very low density fluid.

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  27. Really interesting post and I like the way that you have described the evidence leading to the conclusion that dark matter exists. It leads me to think that in such a case, where the existence of something is “inferred” from evidence, such as the maintenance of stars’ rotational speed around their galactic centre, and the gravitational lens effect that you describe, is it possible that future discoveries relating to these phenomena may cause a revision of the Dark Matter theory – or do you think that it is pretty much proved to exist – we just haven’t developed the means to observe it directly? – fascinating subject. Thanks!

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  28. Reblogged this on Alan Olee Book Report and commented:
    Dark matter interests me – and this is a great post. So there’s this certain something out there, that just has to be there, made up of stuff we can’t understand at all. Amazing how much we know; and how much we don’t.

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      1. One of the things I got to thinking about wrt dark matter, is where is it specifically? Or is it posited that it is diffused around galaxies to provide the necessary binding gravitational force. Or is it thought that it is concentrated at the center of galaxies? Do we think there is any of it in our solar system?

        And are there any current missions we have, I’m thinking Dawn and News Horizons, that may shed some light on dark matter?

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        1. The general view is that dark matter is much more thinly spread out than ordinary matter.
          This is illustrated in the diagram in my post, which shows a typical spiral galaxy surrounded by an invisible dark matter halo.
          Dark matter is likely to be composed of one or more species of sub-atomic particles that interact very weakly with ordinary matter. Particle physicists have many possible candidates for the dark matter particles but non have ever been detected by any particle accelerator experiments. experiment and, because they interact so weakly with ordinary matter, they have never been directly observed

          The Science Geek

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