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What is Dark Matter?

  1. Jun 11, 2003 #1
    So, it seems that Dark Matter now takes up most of space. What does that mean? What exactly is Dark Matter and why did we miss it before? How will change astronomy?

    Educate me on this please.:smile:

  2. jcsd
  3. Jun 11, 2003 #2


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    The second part of your question is kinda the answer to the first; dark matter is matter out there that we can't see, or haven't seen yet. We know it's out there because the movement of the matter we can see shows the presence of gravitational attraction from invisible sources. For example, the movement of most gallaxies shows that they have far more mass than is accounted for by the matter we can see. Also, some stars are seen to be orbitting around some invisible partner.

    These "invisible partners" in binary systems are usually assumed to be cold, dark dwarf-stars, or sometimes black holes. The gallactic motion problem is still a matter of some debate, however. Some astronomers theorise particles called WIMPS (Weakly Interacting Massive Particles), that are hard to detect because they don't "collide" with the objects through which they pass. The ever-elusive neutrino is now known to have some mass, and may account for a large portion of the dark matter in the universe.

    Clouds of dust and gasses between the stars almost certainly contribute to the over-all mass of the cosmos as well.
  4. Jun 11, 2003 #3


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    The stuff that takes up most of space is made up of (according to current understanding) two things, dark matter (30%) and dark energy (60%). Scientists don't know what either of them are, although there are a lot of ideas out there.

    The existence of dark matter has been known for quite a while (30 years?), based on the fact that stars in the galaxies are orbiting the centers too fast to be held in place by the amount of ordinary matter present. About all that is known about dark matter is negative. It doesn't react with ordinary matter or photons.

    Dark energy is a more recent discovery (1998??). The observation which led to its conjectured existence is the speed up of the expansion of the universe. It seems to be related to the energy of the vacuum and/or Einstein's cosmological constant.
  5. Jun 11, 2003 #4
    i don't mean to hijack, but is dark matter the same thing as exotic matter?
  6. Jun 12, 2003 #5


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    No, although most proposed forms of exotic matter would be considered dark matter, dark matter is not confined to those types of matter which would be called "exotic". Exotic matter would be forms of matter that are very unusual. Dark matter includes such things as planets, asteroids, black holes, cold dark dwarf stars, intersteller dust and gas clouds, and anything else that we can't see just by looking through a tellescope.
  7. Jun 12, 2003 #6


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    A better way of characterizing this stuff is to distinguish between baryonic matter (i.e. made up of protons, neutrons, and electrons), which accounts for somewhere between 5% and 10% of the stuff of the universe, and non-baryonic matter (known from gravitational effect), which accounts for about 30% of the stuff. The non-baryonic matter could just as well be called exotic.
  8. Jun 13, 2003 #7
    OK here is a weird thought

    we have matter and anti-matter, could there be a third type of "neutral matter" that doesnot interact with eathor normal matter or anti-matter but is made of quarks also???

    could this stuff also account for the normal matter bias we "see" in the univerce now.

    is there a anti-matter neutron?
    how is it's in it's interactions with normal matter?? ie does a anti-neutron distroy a normal neutron like a postron-electron will act???

    btw are there free neutrons???
    or can we even detect a free neutron??
    how are they accounted for????
    could there be massive amounts of free neutrons???
  9. Jun 13, 2003 #8
    Call me ignorant (because I am or I wouldn't be asking this question), but what kind of proof do we have that anti-matter exist or is it just theory at this stage? If it does exist, what type of use can it be applied to in science?

    Thanks by the way, from everyone who contributed information on the Dark Matter question. I'm still a little lost on the subject and still have other questions though.

    1) If a huge percentage of matter in space is made of dark matter, would it be suffice to say that we are swimming in it?
    2) Are there parts of the universe that are simply absent of matter? (I guess we call that space right?)
    3) Is it possible but that an absence of matter doesn't really exist?

    Anybody have any info/ theories on these questions?
  10. Jun 13, 2003 #9


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    yes, just as the neutron is composed of one Up Quark (with a charge of +2/3) and two Down (each with a charge of -1/3), so there are antineutrons composed of one Anti-Up (-2/3) and two Anti-Down (+1/3). Same neutral charge, but with opposite spin.
    Although free neutrons have been observed, they probably don't exist in significant amounts in nature, because they only have a life of about 15 minutes on average. At least, I think it's about 15 minutes; it's really short, any rate.
  11. Jun 13, 2003 #10


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    Yes, antimatter has been observed in the lab. One scientific use for it is being applied by researchers at CERN. Last September, they created a batch of anti-hydrogen (one positron orbiting one antiproton). This is a relatively stable form of antimatter that can be observed over time and studied. If these atoms behave as predicted, it will be a strong confirmation of the Standard Model.
  12. Jun 16, 2003 #11
    A friend of mine and I have talked about Dark matter and Dark energy a while ago and theorised that:

    Dark mass = The mass in the current universe in a different quantum state (or possibly a parallel universe/dimension)

    Dark energy = the gravitation effect from the mass in a different quantum state (or the mass from the parallel universe/dimension)

    Any thoughts?
  13. Jun 16, 2003 #12


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    Also, we now have good evidence that most of the Dark Matter in the universe is Cold Dark Matter.
  14. Jun 16, 2003 #13
    All we really have evidence of

    is the fact that the large scale structures in the Universe don't fit our current model of gravity and matter. At the present time only one possible explaination has been proffered, that it concerns some sort of invisible matter. Other possibilites have simply not been considered.

    What we have is failure to imaginate! [oo] [oo] [oo] [oo] [oo]
  15. Jun 16, 2003 #14


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    This is not true. Milgrom has proposed a theory called MOND (modified Newtonian dynamics) which accounts for the galaxy motion problem without dark matter. It is somewhat ad hoc and difficult to test. It has been described in Scientific American tnis year.
  16. Jun 17, 2003 #15


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    Yeah. The problem with MOND is that while it is very accurate, it has practically no theoretically grounding and is usually regarded as a numerological fudge made just to describe our observed statistics.

    BTW, how does MOND respond to the recent WMAP map of cosmic microwave background, which specifies a set percentage on non-baryonic mass?
  17. Jun 17, 2003 #16
    Very good info

    I'll have to find that Sci Am article. Here's a good forum article with replies that I found.


    There was another nice page but it kept crashing my browser, so I didn't put it up.

    It's true that it is ad hoc, but so was Newton's Gravity. And it should be no more difficult to fit MOND into a larger framework than Newton's ideas.

    You can find plenty on the web, just put "mond theory" in your browser.

    Rely to FZ+ questions; Many worthwhile theories started out as "numerical fudge" and didn't find a basis until later.

    The jury is still debating on the WMAP, neither possibility seems to be ruled out.
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