Silly Dark matter question: why exactly is it necessary?

  1. Thinking about how little we know even about the amount of mass in our own solar system (Oort Cloud etc), could the discrepancies in mass simply be a ton of space junk (asteroids, planets etc) and even objects like cold neutron stars that are all very explainable objects in terms of their composition for the most part but are just really difficult to see?

    Now don't get me wrong, I would find it highly improbable that the types of particles that we commonly interact with and form the spectrum of chemical elements that we are accustomed to is the only type of particular matter in the universe, or even the only type that can form larger "chemical systems". I am skeptical of our ability to account for all the mass of the common types of matter, how this non-luminous is distributed in the galaxy (are there some kind of galaxy formation dynamics that cause more or less of the non-luminous mass- relative to star material- to be distributed at the edge etc?)

    I suppose I could keep going but I'm all questions and no answers. I'm just hoping someone can enlighten me to some of the stronger arguments for the existence of exotic dark matter vs. hard-to-see regular matter.
  2. jcsd
  3. cepheid

    cepheid 5,189
    Staff Emeritus
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  4. phyzguy

    phyzguy 2,497
    Science Advisor

    Another strong argument against the dark matter being made out of ordinary atoms is the predictions of BBN (Big Bang Nucleosynthesis). Basically, if the unseen matter were made out of baryons (i.e. ordinary atoms), then as the big bang cooled, nuclear fusion would have created a much higher number of heavy elements (He-4, D, He-3, Li) than we see. Since these nuclear fusion cross sections can be measured very accurately in the lab, we can predict very accurately how much of these elements there should be. If we assume the amounts of dark matter and ordinary matter given by the observations of the CMB, then the predictions of the abundances of the elements heavier than hydrogen agree very well with observations, which is a strong argument in favor of the whole model. Ned Wright has a really nice web site on BBN here.
  5. The solar system is not a very good comparison since the mass of our solar system appears to be comprised almost entirely of ordinary, known objects like the sun and the planets and very little dark matter (I am uncertain if it is even detectable at all).

    Most of the dark matter is in the halo of the galaxy and was discovered because of the relatively flat radial rotation curve of objects orbiting the center of the Milky Way.

    Some dark matter is indeed ordinary baryonic matter like neutron stars and there are probably some astronomers who still believe in dark matter being comprised of MACHOS, but they are few and far between these days.

    There are a whole lot of reasons for this skepticism. For instance, neutron stars are not truly "dark" and would be detectable due to gravitational lensing. Furthermore, since most of the dark matter is in the halo, we would need to explain why there would bee so many massive, dead stars in the halo and almost no low-mass, red dwarfs.
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