July 30, 2009 | David F. Coppedge

Dark Matter: Where Is It?

If physicists and astronomers are going to continue to tell us that 95% of the mass in the universe is hidden in some unobservable dark matter, they had better find it soon.  Two articles on PhysOrg (#1, #2) reported on continuing efforts to find the elusive stuff – if it exists at all.  The first article begin quizzically,

95%.  That is the percentage of the known Universe that is missing.  As in it is not there.  Or at least if it is there, we can’t see it.  We call this unseen stuff “dark matter”.  That has been well known for sometime.  What is trickier in answering is why?  Why is it that 95% of the universe is made up of this so-named “dark matter?”  An even trickier question is where?  As in where is this dark matter?  It is those two questions that have plagued physicists for decades.  Dark matter, by its own definition cannot be seen, hence its name.  So how do we “see” it, how do we know “where” to look?

The article announced that “for the first time, a team of physicists has gathered evidence.”  What qualifies as evidence, though, may be in the eye of the beholder.  What some physicists found was high-energy positrons from space.  Any connection to dark matter (in this case, weakly-interacting massive particles, or WIMPs) is highly theory-dependent.
    The second article did not claim evidence – just the desire to find it.  A Columbia physicist put it this way: “Sometimes I think of dark matter as a mysterious woman with her face covered by one of those beautiful Venetian masks.  All of us experimentalists are driven by one desire: to uncover that face.”
    Researchers are spending millions of dollars looking for something that may not exist.  There are only indirect reasons for suspecting its existence: theories that require it, and galaxy clusters that would have disrupted from their own internal motions if they are as old as claimed.

How is this different, functionally, from invoking miracles or occult substances to keep one’s pet belief intact?  Keep track of this dark matter story.  We’ll have to see if it becomes a 21st century analogue of alchemy.

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Categories: Cosmology, Physics

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