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Most people do not understand "dark matter" because they do not understand astronomy. Astronomy would be better defined as the study of celestial objects by th

Most people do not understand "dark matter" because they do not understand astronomy. Astronomy would be better defined as the study of celestial objects by the collection of photons, and inferential calculations made upon those collections. Astronomers collect photons from stars, which is to say "matter that gives off light." In a practical sense, the Earth is made of dark matter, because it does not shine by its own light (okay, if you are near enough, shielded from the Sun, and looking in infrared, if you like splitting hairs). Dark matter need not be particularly exotic, just ... almost hard to find by definition. Dark matter has already had some success. Just as an example, the discovery of Neptune -- made entirely through dark matter. We couldn't see it obviously, too much space to search over, but some kind of gravitational disturbance gave suggestions as to where to look. Boom, a whole planet. That is quite literally search via dark matter: it doesn't shine, so we must look for its knock-on effects. Easy enough in the local neighborhood, but at a galactic scale, it is much much harder. And I wouldn't expect anything quick, either. If you want a fascinating journey through astronomy, look up the history of the various estimations of the "standard candle," used as a way to gauge distance. This took quite a lot of time to bounce back and forth between numbers.

breezeTrowel 5y ago
This is incorrect. Yes, Dark Matter is matter that does not emit light but it does not absorb nor reflect light as well. In other words, it does not interact with the electromagnetic force. That makes it extremely exotic. And, no, the Earth is not made of dark matter.
- changoplatanero 5y ago
  missed opportunity to call it stealth matter instead of dark matter
- at_a_remove 5y ago
  That's a rather newer definition. The concept has been around longer than the name, which is why I mentioned Neptune. The original definition was literally anything where the presence is puzzled out from its gravitational attraction instead of than its luminosity. Hence why MACHOs (MAssive Compact Halo Objects) were under consideration for being dark matter. They would consist of brown dwarfs, planets, and the like. They are currently out of vogue for not supplying enough gravitation, but that matter had no special properties to it. Not exotic at all. You can look this up. I sort of feel the frustration here of people on Wikipedia trying to correct something they actually know about.
  - kbelder 5y ago
    It's interesting the way language works, and specifically naming. It's almost magic. There's various places where gravitational effects don't seem to line up with the distribution of mass that we see or expect. Something is generating gravity, that we can't see, in places we didn't expect. Call it 'dark' matter, because it's not shiny. But now that it has a name, it's treated a bit like a proper noun. Dark Matter. We do some observations that probably rule out simple nonluminous planetoids or dark stars. Whatever is causing the galactic rotation discrepancies doesn't seem to be normal matter. That translates into: 'Dark Matter' isn't normal matter. As if it is a certain thing, and not a reference to an unknown set. By naming it, and talking about it, even though we don't understand it, we run the risk of attaching all sorts of presumptions to its identity. We're prematurely normalizing a certain way of thinking about it. We may have done a similar thing to quantum mechanics. (I may not be making much sense here, it's just a thought that struck me and I'm working through it.)
Cd00d 5y ago
With a molten core I'd be surprised if the earth didn't emit it's own light.
- vecter 5y ago
  Anything with a positive temperature radiates photons. The Earth probably does in the infrared spectrum.
  - perardi 5y ago
    Haha, it would be a shock if it didn’t. That whole “black body radiation” thing. And we are imaging the Earth in infrared. Well, infrared from non-geologic sources, but oh man are we sure imaging in infrared. https://en.wikipedia.org/wiki/Suomi_NPP https://en.wikipedia.org/wiki/Suomi_NPP
  - at_a_remove 5y ago
    Precisely why I said "(okay, if you are near enough, shielded from the Sun, and looking in infrared, if you like splitting hairs)" ...
Melatonic 5y ago
We have known about Neptune far longer than Dark Matter - no idea what you are talking about here. I mean we have great photographs of the planet itself - obviously it reflects light !
- at_a_remove 5y ago
  "Light matter" is basically stars. Now, sometimes bodies reflect light (which is why we can see the Moon or Neptune). Bu before the name "dark matter" existed, the practice of it was evident. How did they discover Neptune? Gravitationally, not by seeing it first via a telescope. Its existence was inferred by the orbits of other planets, namely a gravitational disturbance suggesting a big glob of matter we had not collected photons from. But we did not discover Neptune by collecting its photons. We had to infer that some glob of matter was present by gravitation. Then we pointed telescopes in the vicinity of prediction until this was confirmed. Going further, we use these collected to infer all kinds of things -- a consistent dimming of a star in a certain period might suggest a planet occluding the star in an orbit coplanar to ours, which is our current method of locating exoplanets, but that wouldn't count. You might recall how the neutrino was originally simply a placeholder, and actual detection came later. Similarly, the coining of the phase dark matter was simply as a contrast and a placeholder: we know something is there, but it ain't stars (light matter). Historically, many suggestions were made, and quite a few of them weren't exotic: MACHOs (Massive Compact Halo Objects), intergalactic gas, and so on. WIMPs would classify as a candidate for dark matter that would be somewhat exotic, so would axions. Frustratingly, as the article suggests, we can do a lot of things to rule out (or more typically, set a maximum contribution as a fraction from each candidate), but we have little ruled in. We don't even have detection of axions yet, and those have been in the running for decades. This is the sort of stuff that lured me into getting a physics degree.

breezeTrowel 5y ago

This is incorrect. Yes, Dark Matter is matter that does not emit light but it does not absorb nor reflect light as well. In other words, it does not interact with the electromagnetic force. That makes it extremely exotic. And, no, the Earth is not made of dark matter.

changoplatanero 5y ago
missed opportunity to call it stealth matter instead of dark matter
at_a_remove 5y ago
That's a rather newer definition. The concept has been around longer than the name, which is why I mentioned Neptune. The original definition was literally anything where the presence is puzzled out from its gravitational attraction instead of than its luminosity. Hence why MACHOs (MAssive Compact Halo Objects) were under consideration for being dark matter. They would consist of brown dwarfs, planets, and the like. They are currently out of vogue for not supplying enough gravitation, but that matter had no special properties to it. Not exotic at all. You can look this up. I sort of feel the frustration here of people on Wikipedia trying to correct something they actually know about.
- kbelder 5y ago
  It's interesting the way language works, and specifically naming. It's almost magic. There's various places where gravitational effects don't seem to line up with the distribution of mass that we see or expect. Something is generating gravity, that we can't see, in places we didn't expect. Call it 'dark' matter, because it's not shiny. But now that it has a name, it's treated a bit like a proper noun. Dark Matter. We do some observations that probably rule out simple nonluminous planetoids or dark stars. Whatever is causing the galactic rotation discrepancies doesn't seem to be normal matter. That translates into: 'Dark Matter' isn't normal matter. As if it is a certain thing, and not a reference to an unknown set. By naming it, and talking about it, even though we don't understand it, we run the risk of attaching all sorts of presumptions to its identity. We're prematurely normalizing a certain way of thinking about it. We may have done a similar thing to quantum mechanics. (I may not be making much sense here, it's just a thought that struck me and I'm working through it.)

Cd00d 5y ago

With a molten core I'd be surprised if the earth didn't emit it's own light.

vecter 5y ago
Anything with a positive temperature radiates photons. The Earth probably does in the infrared spectrum.
- perardi 5y ago
  Haha, it would be a shock if it didn’t. That whole “black body radiation” thing. And we are imaging the Earth in infrared. Well, infrared from non-geologic sources, but oh man are we sure imaging in infrared. https://en.wikipedia.org/wiki/Suomi_NPP https://en.wikipedia.org/wiki/Suomi_NPP
- at_a_remove 5y ago
  Precisely why I said "(okay, if you are near enough, shielded from the Sun, and looking in infrared, if you like splitting hairs)" ...

Melatonic 5y ago

We have known about Neptune far longer than Dark Matter - no idea what you are talking about here. I mean we have great photographs of the planet itself - obviously it reflects light !

at_a_remove 5y ago
"Light matter" is basically stars. Now, sometimes bodies reflect light (which is why we can see the Moon or Neptune). Bu before the name "dark matter" existed, the practice of it was evident. How did they discover Neptune? Gravitationally, not by seeing it first via a telescope. Its existence was inferred by the orbits of other planets, namely a gravitational disturbance suggesting a big glob of matter we had not collected photons from. But we did not discover Neptune by collecting its photons. We had to infer that some glob of matter was present by gravitation. Then we pointed telescopes in the vicinity of prediction until this was confirmed. Going further, we use these collected to infer all kinds of things -- a consistent dimming of a star in a certain period might suggest a planet occluding the star in an orbit coplanar to ours, which is our current method of locating exoplanets, but that wouldn't count. You might recall how the neutrino was originally simply a placeholder, and actual detection came later. Similarly, the coining of the phase dark matter was simply as a contrast and a placeholder: we know something is there, but it ain't stars (light matter). Historically, many suggestions were made, and quite a few of them weren't exotic: MACHOs (Massive Compact Halo Objects), intergalactic gas, and so on. WIMPs would classify as a candidate for dark matter that would be somewhat exotic, so would axions. Frustratingly, as the article suggests, we can do a lot of things to rule out (or more typically, set a maximum contribution as a fraction from each candidate), but we have little ruled in. We don't even have detection of axions yet, and those have been in the running for decades. This is the sort of stuff that lured me into getting a physics degree.