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Astronomy

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  • $\begingroup$ Do we know why when the galaxy formed the dark matter didn't get distributed similarly to the regular matter? Why is it out there in halos? Shouldn't gravity pull it in like normal matter into clumps and stars, etc.? $\endgroup$
    – joseph.hainline
    Commented Feb 19, 2017 at 23:18
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    $\begingroup$ Regular matter in the form of e.g. gas clouds can collide and lose energy and angular momentum, so that the gas will end up on smaller orbits closer to the center of the (proto-)galaxy. Gas clouds are also supported against self-gravity by internal pressure, which depends on (among other things) temperature; since gas can cool by radiation, gas clouds can end up with lower pressure and thus collapse to form clumps and stars. $\endgroup$
    – Peter Erwin
    Commented Feb 20, 2017 at 0:04
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    $\begingroup$ So the answer in a sense is: dark matter and regular matter started out with similar distributions (and collapsed to form halos in the early universe); but regular matter in the form of gas can lose orbital energy and thus collapse further in ways dark matter can't. $\endgroup$
    – Peter Erwin
    Commented Feb 20, 2017 at 0:06