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Re: Dark Matter

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Posted by Ouranou on November 24, 1998 12:17:14 UTC

: : Dark matter does exist we can't see it because somehow it does not reflect light it only absorbs it. We can see its pressence though its gravitational effect.

: So, then, are you saying that dark matter is synonymous with what we commonly refer to as a black hole? Forgive any ignorance on my part, but I have never, in my studies, run accross that term before.

Not exactly, though a black hole could qualify as dark matter. Just imagine it this way, we see stars and such becuase they either give off or reflect light, dark matter does not give off any light and does not rflect light (or any EM radiaiton) very well, so we cannot see it with normal or radio telescopes. This does not mean black holes per-se, dark matter could be brown dwarves or dense gas clouds not yet stars, or nuetron/proton stars too faint to see, basically anything fainter than a normal star. One of the reasons that we predict it's existance has to do with our measurements of the rotational velocity of the galaxy. It's very complicated, but we have ways of detecting the velocity of matter in the galaxy as you get farther from the center. According to Kepler's 3rd law; as the mass of the galaxy diffuses over the distance from the center, the outer rim should have a much lower velocity than the center. However, the data we have collected tells us that the velocity of the outer edges of the galaxy is not nearly as low as it should be, it may actually even increase velocity as the distance from the center increases. Therefore it is predicted that there is MUCH more mass in the galaxy than we can detect directly from EM colleting devices. About 90% of the galaxy is supposed to be dark matter under this theory. The presence of many many black holes could be used to explain this extra mass, but you must account that for a black hole to form there must be a super nova explosion, and such a large scale presence of SN remnants would leave a much higher metal composition in the galaxy than we have evidence for. Thus the exact nature of dark matter remains a mystery.

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