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I am not an astrophysicist... but does this mean dark matter has finally been "seen"? That's a big deal, right?

brianyu8 9y ago
I was also confused about this. However the article states that half of the normal matter (not dark matter) thought to exist has been detected. Basically we just found the missing normal matter and have not yet found dark matter.
mcbutterbunz 9y ago
No, this is just the half of the 5% of regular matter that was "missing".
warent 9y ago
I think it's actually saying that we discovered missing matter that isn't dark matter. So now all that's remaining unaccounted-for is dark matter (and dark energy) Disclaimer: also not an astrophysicist
JohnBooty 9y ago
EDIT: My understanding is incorrect. See the replies to the post for a correction. No. They're saying that 50% of the "dark matter" isn't dark, it's just regular matter that we weren't able to see. The density of this stuff (filaments of gas stretching between galaxies) is so low that it is really, really, really hard to detect. "Dark matter" theories posit that the missing matter isn't just regular matter that's hard to see; they posit that this dark matter is a fundamentally different... something. I don't believe this announcement addresses some of the other oddities (possibly) explained by dark matter, such as the rotation of galaxies: https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve
- timthorn 9y ago
  > They're saying that 50% of the "dark matter" isn't dark, it's just regular matter that we weren't able to see. No, the dark matter proportion hasn't changed. There was missing "regular" matter that has now been accounted for.
  - JohnBooty 9y ago
    Thanks for the correction. I updated my post with a note pointing out that it's wrong.
- justin_vanw 9y ago
  No, this has nothing to do with dark matter. Dark matter has to be in galaxies because the reason we think it exists at all is because galaxies spin 'too fast', so there is more mass in them and therefore more gravity than can be accounted for. This article is talking about gas between galaxies.
  - k__ 9y ago
    Isn't it more probable that the formulars to calculate the spin are wrong?
    - planteen 9y ago
      That's a theory called Modified Newton Dynamics (MOND). https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics
      - wbhart 9y ago
        This theory (when appropriately hacked) describes rotation curves of galaxies extremely well. Unfortunately it doesn't work at larger scales. There are many nice videos on YouTube by Sean Carroll on MOND, which explain it's limits, albeit for the fairly educated layperson.
    - justin_vanw 9y ago
      Its possible but there is a lot of indirect evidence for dark matter, it fits observation well even if we cant prove it. When gravity calculations give an answer that conflicts with observation, sometimes the equation for gravity ends up being wrong (orbit of mercury) and sometimes you find missing mass (discovery of neptune). I don't know what more probable means in this context, the experts are at least as smart as you and me, they favor some kind of missing mass.
  - wbhart 9y ago
    But we know they "spin too fast", by measuring the amount of observable matter and then measuring the "rotation curve" of the galaxy. However, the basic point that this is not half the dark matter, is correct, (based on our current understanding). Even if we can't see ordinary matter, we should be able to see its effect in some way other than gravitational interaction. Dark matter is only seen by its gravitational interaction. It doesn't interact electromagnetically.
sanxiyn 9y ago
It's not a dark matter.
nikdaheratik 9y ago
Not the same as dark matter. This is normal matter that is just very hard to see because it is laid out in thin tendrils of dense subatomic particles that are stretched between galaxies. Dark matter is supposed to only have weak interactions with other particles, basically only gravity and not EM or other forces. Which is why we haven't been able to really detect it and can only deduce its existence.
- colechristensen 9y ago
  "dark matter" is a blanket term for stuff we have had a hard time seeing but we very strongly believe is there Some of the candidates for where all the missing mass is, is poorly interacting matter
- throwaway613834 9y ago
  > basically only gravity and not EM or other forces I don't understand how physicists make any sense of this in any kind of theory. If you had enough dark matter sitting in some spot that could turn into a star, suddenly the claim is any ordinary matter around it would stay near absolute zero no matter how much nuclear fusion was going on at the same spot? How does that work? Or would dark matter just somehow resist even interacting with itself to create friction, etc.?
  - justin_vanw 9y ago
    Friction, and really any kind of 'contact' between things is just electrons repelling. Some ideas for dark matter don't interact in any way other than gravity, so your idea is pretty mainstream on that, but there's basically no evidence that dark matter exists in some specific form, and in fact every experiment seems to constrain the possible forms it could have more and more. It's still possible it exists, but I would suspect that whatever the explanation is for the 'missing' matter will be pretty revolutionary.
  - nikdaheratik 9y ago
    We don't know much about what it does do, only what it doesn't do. Scientists have deduced its existence by measuring the speed of the expansion of the universe and determining that the mass that we can see based on the rotation of galaxies, etc. can not account for the speed of the expansion. There should be way more mass out there than there is. We also know that it doesn't seem to react with light otherwise it would block out the stars from other galaxies as well as the cosmic background radiation. It's difficult to tell how it reacts with ordinary matter because we can't see it, but the assumption is that, since its got such a large gravitational effect, it must not react much at all otherwise it would dominate everything visible since it's 90% of the matter in the known universe. It's difficult to make sense of what this means, but all of the other theories that explain the rotation of galaxies and the expansion of the universe don't fit very well either.
InclinedPlane 9y ago
No, this is about baryonic (normal) matter entirely. We basically have different ways of determining what the matter makeup of the Universe is which are independent of direct observations. Those techniques actually indicated a higher proportion of atomic (baryonic) matter in the Universe than had been directly detected so far, while still indicating that the vast majority of the matter/energy balance of the Universe is in the form of dark energy (which we are pretty in the dark on its composition) and "cold dark matter" (which has been narrowed down to being primarily made up of "weakly interacting massive particles" of a type or types that are currently outside our understanding of particle physics). Specifically, those lines of evidence say that the expected balance of the density of matter/energy on cosmological scales comes out to 69% dark energy, 26% cold dark matter, and the remainder (actually a bit under 5%) being baryonic matter. However, we've only detected about half of that expected 5% density of matter (in the form of stars & planets, black holes, giant interstellar gas and dust clouds, etc.) This recent discovery shows that much of the undetected baryonic matter is in the form of huge warm gas clouds surrounding galaxies.
guelo 9y ago
Adding to the other comments that this "regular" matter wasn't really missing. All the models predicted that it was there in the filaments between galaxies but we had no way to detect it. These experiments are just a confirmation of the current consensus and strengthens trust in our models.
archgoon 9y ago
From the second paragraph of the article: "You have probably heard about the hunt for dark matter, a mysterious substance thought to permeate the universe, the effects of which we can see through its gravitational pull. But our models of the universe also say there should be about twice as much ordinary matter out there, compared with what we have observed so far." This could have been worded slightly differently to emphasize that the second sentence is contrasting with the first, but no; this is about regular matter, not Dark Matter.

I was also confused about this. However the article states that half of the normal matter (not dark matter) thought to exist has been detected. Basically we just found the missing normal matter and have not yet found dark matter.

mcbutterbunz 9y ago

No, this is just the half of the 5% of regular matter that was "missing".

warent 9y ago

I think it's actually saying that we discovered missing matter that isn't dark matter. So now all that's remaining unaccounted-for is dark matter (and dark energy) Disclaimer: also not an astrophysicist

JohnBooty 9y ago

EDIT: My understanding is incorrect. See the replies to the post for a correction. No. They're saying that 50% of the "dark matter" isn't dark, it's just regular matter that we weren't able to see. The density of this stuff (filaments of gas stretching between galaxies) is so low that it is really, really, really hard to detect. "Dark matter" theories posit that the missing matter isn't just regular matter that's hard to see; they posit that this dark matter is a fundamentally different... something. I don't believe this announcement addresses some of the other oddities (possibly) explained by dark matter, such as the rotation of galaxies: https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve

timthorn 9y ago
> They're saying that 50% of the "dark matter" isn't dark, it's just regular matter that we weren't able to see. No, the dark matter proportion hasn't changed. There was missing "regular" matter that has now been accounted for.
- JohnBooty 9y ago
  Thanks for the correction. I updated my post with a note pointing out that it's wrong.
justin_vanw 9y ago
No, this has nothing to do with dark matter. Dark matter has to be in galaxies because the reason we think it exists at all is because galaxies spin 'too fast', so there is more mass in them and therefore more gravity than can be accounted for. This article is talking about gas between galaxies.
- k__ 9y ago
  Isn't it more probable that the formulars to calculate the spin are wrong?
  - planteen 9y ago
    That's a theory called Modified Newton Dynamics (MOND). https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics
    - wbhart 9y ago
      This theory (when appropriately hacked) describes rotation curves of galaxies extremely well. Unfortunately it doesn't work at larger scales. There are many nice videos on YouTube by Sean Carroll on MOND, which explain it's limits, albeit for the fairly educated layperson.
  - justin_vanw 9y ago
    Its possible but there is a lot of indirect evidence for dark matter, it fits observation well even if we cant prove it. When gravity calculations give an answer that conflicts with observation, sometimes the equation for gravity ends up being wrong (orbit of mercury) and sometimes you find missing mass (discovery of neptune). I don't know what more probable means in this context, the experts are at least as smart as you and me, they favor some kind of missing mass.
- wbhart 9y ago
  But we know they "spin too fast", by measuring the amount of observable matter and then measuring the "rotation curve" of the galaxy. However, the basic point that this is not half the dark matter, is correct, (based on our current understanding). Even if we can't see ordinary matter, we should be able to see its effect in some way other than gravitational interaction. Dark matter is only seen by its gravitational interaction. It doesn't interact electromagnetically.

sanxiyn 9y ago

It's not a dark matter.

nikdaheratik 9y ago

Not the same as dark matter. This is normal matter that is just very hard to see because it is laid out in thin tendrils of dense subatomic particles that are stretched between galaxies. Dark matter is supposed to only have weak interactions with other particles, basically only gravity and not EM or other forces. Which is why we haven't been able to really detect it and can only deduce its existence.

colechristensen 9y ago
"dark matter" is a blanket term for stuff we have had a hard time seeing but we very strongly believe is there Some of the candidates for where all the missing mass is, is poorly interacting matter
throwaway613834 9y ago
> basically only gravity and not EM or other forces I don't understand how physicists make any sense of this in any kind of theory. If you had enough dark matter sitting in some spot that could turn into a star, suddenly the claim is any ordinary matter around it would stay near absolute zero no matter how much nuclear fusion was going on at the same spot? How does that work? Or would dark matter just somehow resist even interacting with itself to create friction, etc.?
- justin_vanw 9y ago
  Friction, and really any kind of 'contact' between things is just electrons repelling. Some ideas for dark matter don't interact in any way other than gravity, so your idea is pretty mainstream on that, but there's basically no evidence that dark matter exists in some specific form, and in fact every experiment seems to constrain the possible forms it could have more and more. It's still possible it exists, but I would suspect that whatever the explanation is for the 'missing' matter will be pretty revolutionary.
- nikdaheratik 9y ago
  We don't know much about what it does do, only what it doesn't do. Scientists have deduced its existence by measuring the speed of the expansion of the universe and determining that the mass that we can see based on the rotation of galaxies, etc. can not account for the speed of the expansion. There should be way more mass out there than there is. We also know that it doesn't seem to react with light otherwise it would block out the stars from other galaxies as well as the cosmic background radiation. It's difficult to tell how it reacts with ordinary matter because we can't see it, but the assumption is that, since its got such a large gravitational effect, it must not react much at all otherwise it would dominate everything visible since it's 90% of the matter in the known universe. It's difficult to make sense of what this means, but all of the other theories that explain the rotation of galaxies and the expansion of the universe don't fit very well either.

InclinedPlane 9y ago

No, this is about baryonic (normal) matter entirely. We basically have different ways of determining what the matter makeup of the Universe is which are independent of direct observations. Those techniques actually indicated a higher proportion of atomic (baryonic) matter in the Universe than had been directly detected so far, while still indicating that the vast majority of the matter/energy balance of the Universe is in the form of dark energy (which we are pretty in the dark on its composition) and "cold dark matter" (which has been narrowed down to being primarily made up of "weakly interacting massive particles" of a type or types that are currently outside our understanding of particle physics). Specifically, those lines of evidence say that the expected balance of the density of matter/energy on cosmological scales comes out to 69% dark energy, 26% cold dark matter, and the remainder (actually a bit under 5%) being baryonic matter. However, we've only detected about half of that expected 5% density of matter (in the form of stars & planets, black holes, giant interstellar gas and dust clouds, etc.) This recent discovery shows that much of the undetected baryonic matter is in the form of huge warm gas clouds surrounding galaxies.

guelo 9y ago

Adding to the other comments that this "regular" matter wasn't really missing. All the models predicted that it was there in the filaments between galaxies but we had no way to detect it. These experiments are just a confirmation of the current consensus and strengthens trust in our models.

archgoon 9y ago

From the second paragraph of the article: "You have probably heard about the hunt for dark matter, a mysterious substance thought to permeate the universe, the effects of which we can see through its gravitational pull. But our models of the universe also say there should be about twice as much ordinary matter out there, compared with what we have observed so far." This could have been worded slightly differently to emphasize that the second sentence is contrasting with the first, but no; this is about regular matter, not Dark Matter.