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I What is dark matter?

  1. Jan 12, 2017 #1
    All the ordinary matter we can find accounts for only about 4 percent of the universe. We know this by calculating how much mass would be needed to hold galaxies together and cause them to move about the way they do when they gather in large clusters. Another way to weigh the unseen matter is to look at how gravity bends the light from distant objects. Every measure tells astronomers that most of the universe is invisible.

    It's tempting to say that the universe must be full of dark clouds of dust or dead stars and be done with it, but there are persuasive arguments that this is not the case. First, although there are ways to spot even the darkest forms of matter, almost every attempt to find missing clouds and stars has failed. Second, and more convincing, cosmologists can make very precise calculations of the nuclear reactions that occurred right after the Big Bang and compare the expected results with the actual composition of the universe. Those calculations show that the total amount of ordinary matter, composed of familiar protons and neutrons, is much less than the total mass of the universe. Whatever the rest is, it isn't like the stuff of which we're made.

    The quest to find the missing universe is one of the key efforts that has brought cosmologists and particle physicists together. The leading dark-matter candidates are neutrinos or two other kinds of particles: neutralinos and axions, predicted by some physics theories but never detected. All three of these particles are thought to be electrically neutral, thus unable to absorb or reflect light, yet stable enough to have survived from the earliest moments after the Big Bang.
     
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  3. Jan 12, 2017 #2
    Hi, @Johnsmith123, and welcome to PhysicsForums!
    With all respect, what is the purpose of your post? I do not see any specific question posted. If the question is merely "What is dark matter?" the answer is "we do not yet know". The question of dark matter is one the greatest scientific mysteries of today.
     
  4. Jan 12, 2017 #3

    Delta²

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    This serves as a nice little introduction to dark matter, but some mentor has to move it to the cosmology forum I think.
     
  5. Jan 12, 2017 #4

    Vanadium 50

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    The post was plagiarized from Discover magazine.
     
  6. Jan 12, 2017 #5
    When I read it first, it sounded like it came from somewhere else, but I assumed good faith.
     
  7. Jan 12, 2017 #6
  8. Jan 13, 2017 #7

    Chronos

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    The short answer is dark matter is the stuff that gravitates without emitting any EM radiation.
     
  9. Jan 13, 2017 #8
    Any serious consensus on strangelets as a candidate for dark matter? I've also heard on some recent pop-sci shows that magnetism may explain the stellar and galactic behavior for which dark matter was invented, any real science behind this claim?
     
  10. Jan 13, 2017 #9

    PeterDonis

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    No.

    Which are not valid sources here on PF.
     
  11. Jan 13, 2017 #10
    While we're on the subject, I have a question: I understand dark matter is invoked to explain the observed orbital dynamics of stars within galaxies. Why do we not see its effects within individual star systems? IOW, why isn't the Sun's gravitational attraction higher that we would expect given the amount of observed ordinary matter? If the answer is that dark matter is diffuse, why would that be if it only interacts gravitationally? It seems that dark matter would collect everywhere there is a significant accumulation of ordinary matter.
     
  12. Jan 13, 2017 #11

    PeterDonis

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    Because on the scale of an individual star system, the density of dark matter is too small to affect the dynamics significantly.

    No, it won't. Ordinary matter clumps much more effectively than dark matter does, because ordinary matter can lose energy by emitting EM radiation, which causes it to form tightly bound systems like stars and planets and star systems. Dark matter can only interact gravitationally, and the only gravitational interaction that can cause an isolated system to lose energy and become more tightly bound is the emission of gravitational radiation, which is extremely weak compared to the EM radiation emitted by ordinary matter as it clumps.
     
  13. Jan 13, 2017 #12
    Well, I hadn't considered that. Fascinating, thank you. .
     
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