Oct 10 2017

Half the Matter in the Universe Just Found

filamentsBy now most people are familiar with dark matter – that mysterious substance which has gravity but otherwise does not seem to interact with the normal matter with which we are most familiar. About 27% of the stuff (matter and energy) in the known universe is dark matter, 68% is dark energy, and only about 5% is made of known particles (baryons – protons, neutrons; leptons – electron; and more exotic particles).

We currently don’t know what dark matter is. We know it’s there because we can see its gravitational effect, first noticed because galaxies spin faster than they should. Based just on the gravity from stuff we can see, galaxies should be flying apart. They stick together because there is significantly more gravity than we can account for. There must be additional matter we can’t see, or dark matter.

It is perhaps less well-known that we also haven’t found about half of the normal matter that should exist in the universe. Even if we just consider that 5% that is made of standard particles, about half of it is missing. That is – until now, if recent reports are accurate.

This really wasn’t much of a mystery (not like dark matter) – astronomers suspected that the missing matter was present in the form of diffuse gas between galaxies. There is a lot of space out there, and even a wispy vapor could contain a lot of particles, as much as is contained in all the visible galaxies. The problem is, this thin gas is too wispy to see with conventional means.

Two groups of astronomers, however, have found a way to detect it. The two teams,  one at the Institute of Space Astrophysics (IAS) in Orsay, France, and the other from the University of Edinburgh, used data from the Planck satellite. They also both used a phenomenon known as the Sunyaev-Zel’dovich effect. When the cosmic background radiation passes through hot plasma it tends to brighten a little. Therefore, we can image the temperature of the CBR and use that to map out the hot plasma in the universe.

When the two teams did this they both found that there is hot plasma in the form of filaments that stretch between the visible galaxies. These filaments are 3-6 times denser than the background gas in the universe – which adds enough matter to the visible universe to account for the missing 50% of normal matter.

Astrophysicist Ralph Kraft is quoted as saying:

“This goes a long way toward showing that many of our ideas of how galaxies form and how structures form over the history of the universe are pretty much correct.”

Essentially astronomers pretty much knew this stuff was out there, but these are the first observations to actually demonstrate it. Sometimes new discoveries challenge what we think we know about the universe. And sometime discoveries confirm what we thought we knew. The former tend to get more media and public attention, but it is important to recognize how science progresses in all its facets.

I like to think of this in terms of a jigsaw puzzle analogy. Trying to figure out how the universe works, or any complex scientific question, is like putting together a jigsaw puzzle, without any reference picture, and without any edges, or even knowing how many pieces there are. Sometimes when we find a new piece it fits into the picture we are building. But sometimes the piece doesn’t fit – it expands the puzzle and shows us it is bigger and more complex than we previously thought.

This finding puts a piece right in the middle of our puzzle of the universe, and pretty much right where we thought it should go.

That still leaves 95% of the universe as a mystery. Dark energy and dark matter were definitely pieces that fit outside of the known edges of the picture. We suddenly realized the picture was 20 times bigger than we thought.

However, we should resist the temptation to overhype the significance of the mysterious nature of dark matter and dark energy. Often those who wish to cast doubt (either in general or on some specific area of science) will simplistically assume that because there is something we don’t know that automatically casts doubt one something else we think we do know. This is not necessarily correct, however.

It is possible to have confident scientific knowledge in one area, even if other areas remain unknown. Further, it’s even possible to be confident about one level of knowledge in an area even when deeper questions in that same area are unknown.

For example, we could be very confident that DNA is the primary molecule of inheritance before we understood how it worked. We can be confident that humans and other apes share a recent common ancestor, even before we fully flesh out all the complexity of our ancestry.

This is where the jigsaw puzzle analogy breaks down. As the picture emerges we aren’t just adding new crystal clear pieces. Some of the pieces and the resultant images they contain are blurry or low resolution. As science progresses the picture becomes more clear and more detailed. We are zooming into the picture, not just adding pieces. (So we have to invoke a digital jigsaw puzzle with individual pieces that can vary in terms of their resolution and focus.)

So, for example, when looking at a blurry picture of a tree, at some resolution you can be highly confident that it is, in fact, a tree and nothing else, even before you can see in enough detail to know what kind of tree it is. As the picture becomes clearer, perhaps at some point you can conclude it is a deciduous tree, and then with more detail that it is a maple tree. But there are still many species of maple and it may not be clear which one. Not knowing which species of maple the tree is, however, does not call into question whether or not it is a tree at all.

To bring this back to the current topic – the standard model of particle physics is wildly successful, has made many highly accurate predictions, and is a very useful construct to understand normal matter. The existence of dark matter adds a mystery to our understanding of the universe, but it does not invalidate the standard model.

The missing normal matter was an even smaller mystery. We basically knew it was there and where it was, we just needed to develop a technique for seeing it – and we did. A few more puzzle pieces snap into place with a satisfying click.

16 responses so far

16 thoughts on “Half the Matter in the Universe Just Found”

  1. Kabbor says:

    this thin gas is to wispy to see -> this thin gas is too wispy to see
    Please delete this post, it is only for the select few to see.

  2. Kabbor says:

    I like the jigsaw analogy, but it does indeed have some limitations in describing the process of science. If you think of the individual pieces as being small enough, or the picture is big enough it still mostly works, though then you strain the benefit of the metaphor because it no longer resembles something people find in the physical world.

    Interesting find, I am always left scratching my head at the notion of very hot very diffuse gas in the universe. I would think the heat would radiate away rather quickly from these things, but I don’t have the scientific background to know how these things stay hot over such longs periods.

  3. SquareWheel says:

    A space filament you say? Star Trek was right!


  4. Lobsterbash says:

    Dark energy and dark matter has to be one of the greatest gifts to the scifi genre. Yet it’s disappointing how little mainstream scifi has focused on the stuff.

  5. Pete A says:


    “I would think the heat would radiate away rather quickly from these things”

    The net thermal radiative power of a black body radiator is proportional to the fourth power of: its absolute temperature, minus the absolute temperature of its surroundings. Therefore, a very hot gas will initially cool quickly, but as it cools, its rate of cooling reduces dramatically.

  6. Kabbor says:

    Pete A,
    These things are supposed to be hundreds of thousands to 10 million degrees kelvin so I would think they’d be very much in the ‘cool quickly’ part of the curve.

    I guess it doesn’t make intuitive sense to me that rocky planets seem to cool down relatively quickly in the absence of a sun and yet these gasses in deep space retain significant heat over billions of years. Again, I am not disputing the findings, but it is certainly not intuitive that these things would be so hot. I guess that the incomprehensively vast size of the cosmic filaments make them amazing insulators.

  7. Maculus says:

    I’m not an expert, but I believe a particle needs to interact with another particle in order to emit radiation (lose heat). So a dense cloud would lose heat faster than a wispy cloud because the particles would collide more frequently. (also assuming the cloud isn’t expanding or collapsing)

  8. exoheuri says:

    I remember from way back that huge currents flowed between galaxies.

  9. Pete A says:


    “These things are supposed to be hundreds of thousands to 10 million degrees kelvin so I would think they’d be very much in the ‘cool quickly’ part of the curve.”

    Using linear scales on both the vertical temperature axis and the horizontal time axis: Yes! But it depends on the space-time scale in which you are using the phrase “cool quickly”.

    We know that a volcanic lava flow initially cools ‘quickly’, but it takes ages before it has cooled to the point of being unable to melt the soles of our shoes.

    I think you nailed it with “I guess that the incomprehensively vast size of the cosmic filaments make them amazing”, but I’m reluctant to agree with the your subsequent word “insulators”.

    I’m in no way trying to be argumentative. I’m just hoping to live long enough to see the findings from the forthcoming James Webb Space Telescope.

  10. Pete A says:


    “a particle needs to interact with another particle in order to emit radiation (lose heat)”

    In the case of the thermonuclear radiation emitted by the Sun, that is indeed true.

    However, thermal radiation which loses/emits heat at temperatures vastly lower than millions of kelvin does not require interactions between particles.

  11. Kabbor says:

    Thanks for the explanation about the thermal radiation. Due to this being space plasma and very diffuse the inter-atomic collisions are comparatively few and far between compared to planets. I hadn’t known that it required a collision because that is our everyday existence. I collide with myself all the time, because I’m just dense that way.

  12. Pete A says:


    What I find interesting is that some physicists (including some astrophysicists) frequently remind us that individual particles do not have a temperature because the term “temperature” relates to an object consisting of many particles (solid, liquid, or gas)[1]. E.g., one molecule of water, H₂O, isn’t water, and boiling it will not produce steam. Similarly, a hot air balloon wouldn’t work with one molecule of O₂ or N₂. A gas in a container exerts a pressure due to the collisions between the molecules, but if there’s only one molecule then the only thing it can collide with is the container. And a photon doesn’t have a temperature despite the fact that a stream of photons from a high-power laser can melt steel.

    Plasma is one of the four fundamental states of matter. The intracluster medium is the superheated plasma in a galaxy cluster. It’s density is circa 1,000 particles per cubic metre (1 particle per litre; 10⁻³ particles per cubic centimetre) and the mean free path of the particles is circa one lightyear! Using classical mechanics, each litre of this plasma doesn’t possess a temperature, a pressure, or a volume. Using quantum mechanics, we can estimate the temperature of the plasma from the wavelength spectrum of its photon radiation, but we have to be careful because plasma is far from being a typical black body radiator of photons. We need to treat superheated plasma as interactions between particles and fields, not as mechanical particle collisions. Here’s one such treatment:

    [1]: Although thermodynamic temperature does relate to kinetic energy and potential energy therefore it can be applied to individual particles, I think the frequent reminders that it doesn’t are necessary: because it’s far too easy to inadvertently commit a category error, and/or the fallacy of composition, which leads to false premises then to invalid conclusions.

  13. Kabbor says:

    Hmm… science, yes… category error you say… Are you telling me… I have super powers?

    Kidding aside, I appreciate taking the time to enlighten me and any others that have had fuzzy notions of temperature in space plasma.

  14. Gingerbaker says:

    I’ve got a hypothesis about the rest of the missing matter and it involves clothes dryers and woolen socks.

  15. GingTho says:

    With all this talk of ‘cooling’, I’d just like to point out that there is no such things as cool/cold, it’s just hot and less hot until you get to absolute 0:


  16. Pete A says:


    Cooling is the transfer of thermal energy via thermal radiation, heat conduction or convection. — Wikipedia.

    You seem to be confusing “the transfer of thermal energy” with “temperature”. Furthermore, the terms “hot” and “cold” refer to relative temperature; whereas the term “absolute temperature” is what it says it is.

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