5 ms·

Dark Matter is a working title for about 85% of the stuff in the entire universe. What it is is the question posed by its name but its existence is generally a

Dark Matter is a working title for about 85% of the stuff in the entire universe. What it is is the question posed by its name but its existence is generally agreed upon. Many apologies to any "English is my second language, please have mercy on my internal parser" for that last sentence. Certain properties of galaxies implies that there is more matter in them than is observed directly. The extra stuff could be clotted cream but then I would personally be able to detect it from billions of light years away. We can only really say that gravity is a thing that we think works like ... this (cue pretty piccies from some Holywood films and Hubble etc). Gravity is an emergent property of matter according to Mr Einstein. Take a large collection of matter, say of the order of 100,000 light years across, like our Milky Way or Andromeda, which is spinning and has a measurable distribution of matter. You can predict how it ought to behave. For example it ought to spin at x or be distributed as y etc. However, measurements show that it doesn't. So you take the observed parameters and run your models and equations backwards to get the actual matter that would be involved. It turns out that over four fifths of stuff is currently unobservable, which is a bit embarrassing. A galaxy without Dark Matter is one that doesn't have stuff that we don't even know what it is that it lacks or something. Assuming that stuff is reasonably homogeneously distributed across the universe, this is a very odd ratio of observable to non observable stuff indeed. Hope that clears that up 8)

foobarian 5y ago
Do you know of any sources that rule out regular matter in between celestial bodies that would be sparse enough to be unobservable but massive enough when put together? Like a bunch of cold electrons?
- xenophonf 5y ago
  We would see something like that against the cosmic microwave background. Instead, we see that about 5/6ths of the total matter of the universe interacts with the CMB only through gravity. Also, more hidden (somehow) baryonic matter would change our current theories surrounding Big Bang nucleosynthesis in ways that aren't supported by other observations, like the percentages of (visible) matter like helium and lithium.
- littlestymaar 5y ago
  If my physics class aren't too far: it cannot be charged otherwise it would radiate, it cannot be atoms because it would absorb radiations and it cannot be neutrons because they don't live long enough by themselves. Then it must be something else…
  - ericbarrett 5y ago
    I believe the only two remaining candidates along these lines are MACHOs and relativistic neutrinos ("hot dark matter"), but I don't think either of them are considered likely among mainstream cosmologists anymore.
- gerdesj 5y ago
  I'm an IT consultant who's been reading New Scientist for 30 years, not an Astrophysicist! Now, you mention "matter ... sparse enough ... massive". I think you have answered your own question. The matter in question may be sparse but if there is enough of it, then it will become "massive" and hence detectible. Not sure what a "cold electron" is - it sounds unlikely. Dark Matter is postulated and not a "thing" per se. A lot of good science does seem to imply that 4/5ths of matter is unseen. The key thing I am trying to get across is that our human need to slap a name on something is not appropriate here but we do it anyway. The cool kids with massive budgets and impressive machines in space cannot yet tell you and I what the stuff is but for now we will call it "dark matter". It might sound a bit mad for people more used to dealing with certainties like Civil Engineering (lol - Britannia Bridge in London - resonance). I'm a Civ Eng grad. That's quite a long way of saying "no"!
sfifs 5y ago
So I have one question and maybe the answer has already been determined. I understand we looked at distribution of mass i galaxies and have inferred that for them to keep a stable shape, they need to be spinning at a certain rate which requires more mass than is observed and hence dark matter. Do we know that galaxies are actually spinning in a steady state and are not actually in the process of flying apart? We've been photographing galaxies for only a hundred years or so at high quality. Would we have detected that they're spinning in a steady stare within this period if time?
- alkonaut 5y ago
  Yes, velocities are known from looking at the wavelengths of light (red/blueshift) So you’d have to fit another massive coincidence which is that the stars flying apart are also at the same time changing their emission spectrums to not look like they are.
  - sfifs 5y ago
    Got it. Makes sense - thanks for your reply.
- gerdesj 5y ago
  Although we have only been observing for 100 odd years in our time frame, the things we have observed have been over billions of years. Remember that distance is time! Think about what a light year means - it's the distance that light travels in one year. The Milky Way (the Romans originally named it that - when you get a decent view of it with no light pollution, it does look like a milky road) is roughly 100,000 light years across. Andromeda is about 2.5 million LY away and so any light from it that we see is getting on a bit. Andromeda is counted as part of the "local group" so considered close to us. Really far away stuff is billions of light years away. That new satellite that's all over the news - James Webb - is something else. Hubble was seriously impressive, even with its "squint" (technical hitch that needed correcting shortly after its launch). Hubble has delivered some of the most stunning science and imagery possible and JW will go much, much further. Just you wait until James Webb starts delivering results. It will be awesome, and I don't use words like that lightly.

foobarian 5y ago

Do you know of any sources that rule out regular matter in between celestial bodies that would be sparse enough to be unobservable but massive enough when put together? Like a bunch of cold electrons?

xenophonf 5y ago
We would see something like that against the cosmic microwave background. Instead, we see that about 5/6ths of the total matter of the universe interacts with the CMB only through gravity. Also, more hidden (somehow) baryonic matter would change our current theories surrounding Big Bang nucleosynthesis in ways that aren't supported by other observations, like the percentages of (visible) matter like helium and lithium.
littlestymaar 5y ago
If my physics class aren't too far: it cannot be charged otherwise it would radiate, it cannot be atoms because it would absorb radiations and it cannot be neutrons because they don't live long enough by themselves. Then it must be something else…
- ericbarrett 5y ago
  I believe the only two remaining candidates along these lines are MACHOs and relativistic neutrinos ("hot dark matter"), but I don't think either of them are considered likely among mainstream cosmologists anymore.
gerdesj 5y ago
I'm an IT consultant who's been reading New Scientist for 30 years, not an Astrophysicist! Now, you mention "matter ... sparse enough ... massive". I think you have answered your own question. The matter in question may be sparse but if there is enough of it, then it will become "massive" and hence detectible. Not sure what a "cold electron" is - it sounds unlikely. Dark Matter is postulated and not a "thing" per se. A lot of good science does seem to imply that 4/5ths of matter is unseen. The key thing I am trying to get across is that our human need to slap a name on something is not appropriate here but we do it anyway. The cool kids with massive budgets and impressive machines in space cannot yet tell you and I what the stuff is but for now we will call it "dark matter". It might sound a bit mad for people more used to dealing with certainties like Civil Engineering (lol - Britannia Bridge in London - resonance). I'm a Civ Eng grad. That's quite a long way of saying "no"!

sfifs 5y ago

So I have one question and maybe the answer has already been determined. I understand we looked at distribution of mass i galaxies and have inferred that for them to keep a stable shape, they need to be spinning at a certain rate which requires more mass than is observed and hence dark matter. Do we know that galaxies are actually spinning in a steady state and are not actually in the process of flying apart? We've been photographing galaxies for only a hundred years or so at high quality. Would we have detected that they're spinning in a steady stare within this period if time?

alkonaut 5y ago
Yes, velocities are known from looking at the wavelengths of light (red/blueshift) So you’d have to fit another massive coincidence which is that the stars flying apart are also at the same time changing their emission spectrums to not look like they are.
- sfifs 5y ago
  Got it. Makes sense - thanks for your reply.
gerdesj 5y ago
Although we have only been observing for 100 odd years in our time frame, the things we have observed have been over billions of years. Remember that distance is time! Think about what a light year means - it's the distance that light travels in one year. The Milky Way (the Romans originally named it that - when you get a decent view of it with no light pollution, it does look like a milky road) is roughly 100,000 light years across. Andromeda is about 2.5 million LY away and so any light from it that we see is getting on a bit. Andromeda is counted as part of the "local group" so considered close to us. Really far away stuff is billions of light years away. That new satellite that's all over the news - James Webb - is something else. Hubble was seriously impressive, even with its "squint" (technical hitch that needed correcting shortly after its launch). Hubble has delivered some of the most stunning science and imagery possible and JW will go much, much further. Just you wait until James Webb starts delivering results. It will be awesome, and I don't use words like that lightly.