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Old 06-02-2008, 12:07 PM
bryanmcc bryanmcc is offline
Join Date: Apr 2000
Location: St. Petersberg, FL USA
Posts: 428
Missing baryonic matter

There have been many reports floating around recently about the discovery of a good chunk of the "missing" matter of the Universe. First, some preliminary accounting of the Universe's matter:

74% dark energy
22% dark matter
4% ordinary (or baryonic) matter

OK, easy enough. The recent reports have been stating that only ~50% of "known" baryonic matter had actually been observed, and a chunk of the missing portion has been discovered as a diffuse plasma between galaxy clusters.

Also easy enough. However, what I haven't seen explained is how 50% of "known" baryonic matter could be missing. Isn't missing, invisible matter that we can deduce must be there due to its gravitational influence on galactic rotation curves, etc., exactly what defines dark matter? How is it that we know that the universe is comprised of 22% dark matter, plus 2% baryonic matter that is also, well, dark, but is not "dark"?

I suspect that this has something to do with how WMAP used the CMB to come up with the 74/22/4 breakdown, but it beats me how it's explained...