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How Much Dark Matter Is There?
Cosmologists like to talk about the amount of matter in the universe in terms of a parameter called Omega. Omega is defined so that a closed universe - a universe that is massive enough that it eventually collapses back onto itself - has Omega larger than 1; an `open' universe, one that expands outwards forever, has Omega less than 1; and a `flat' universe, perfectly balanced between the two, has Omega = 1.
The amount of visible matter in the universe is about Omega = 0.05; not very much at all. Theoreticians like to believe that for the Universe, the total of all of the mass is Omega = 1; this would mean that dark matter makes up the other Omega = 0.95 - 95% of the universe would be dark matter! More realistically, there isn't much evidence for Omega being larger than about 0.4; this would make the amount of dark matter be Omega = 0.35.
Even so, that means that 88% of our universe is a complete mystery.
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