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According to current theory, minute fluctuations in cosmic background radiation detected by NASA's Cosmic Background Explorer (COBE) were all that was necessary to seed the structures which would later become galaxies, galaxy clusters, even superclusters. But how did those primordial fluctuations detected by COBE become the gigantic structures we see today?
The inflationary hypothesis, when applied to the standard Big Bang model of cosmology, implies that the average density of the cosmos is very close to or exactly matches the critical density required to balance its expansion.
In other words, Omega equals one. But when all the visible matter is added together, the resulting density is but 5, at most 10 percent of the critical density. Where, and what, is the missing matter?
And then there are the relative motions of galaxies, galaxy clusters and giant superclusters to reckon with. The speeds at which the Milky Way and our nearest neighbor, Andromeda, are rotating demand some extra, unseen matter--otherwise they would simply fly apart. Not only that, but our Local Group of galaxies and the Virgo cluster are hurtling toward some great, unseen "Great Attractor" at more than one million miles per hour!
For decades, astronomers have tried to explain the stupendous velocities they observe. The objects in question simply don't contain enough visible matter to account for the gravitational forces needed to generate these motions.
Clearly there's more out there than meets the eye. That "something," cosmologists believe, is dark matter. No one quite knows what it's made of, but it seems to comprise at least 90 percent of what's out there!
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