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 <sect1 id="ai-darkmatter">
 
 <sect1info>
 <author>
 <firstname>Jasem</firstname>
 <surname>Mutlaq</surname>
 <affiliation><address>
 </address></affiliation>
 </author>
 </sect1info>
 
 <title>Dark Matter</title>
 <indexterm><primary>Dark Matter</primary>
 </indexterm>
 
 <para>
 Scientists are now quite comfortable with the idea that 90% of the
 mass in the universe is in a form of matter that cannot be seen.
 </para>
 
 <para>Despite comprehensive maps of the nearby universe that cover
 the spectrum from radio to gamma rays, we are only able to account of
 10% of the mass that must be out there.  As Bruce H. Margon, an
 astronomer at the University of Washington, told the New York Times in
 2001: <quote>It's a fairly embarrassing situation to admit that we
 can't find 90 percent of the universe.</quote>  </para>
 
 <para>The term given this <quote>missing mass</quote> is
 <firstterm>Dark Matter</firstterm>, and those two words pretty well
 sum up everything we know about it at this point.  We know there is
 <quote>Matter</quote>, because we can see the effects of its
 gravitational influence.  However, the matter emits no detectable
 electromagnetic radiation at all, hence it is <quote>Dark</quote>.
 There exist several theories to account for the missing mass ranging
 from exotic subatomic particles, to a population of isolated black
 holes, to less exotic brown and white dwarfs.  The term <quote>missing
 mass</quote> might be misleading, since the mass itself is not
 missing, just its light.  But what is exactly dark matter and how do
 we really know it exists, if we cannot see it?  </para>
 
 <para>
 The story began in 1933 when Astronomer Fritz Zwicky was studying the
 motions of distant and massive clusters of galaxies, specifically the
 Coma cluster and the Virgo cluster.  Zwicky estimated the mass of each
 galaxy in the cluster based on their luminosity, and added up all of
 the galaxy masses to get a total cluster mass.  He then made a second,
 independent estimate of the cluster mass, based on measuring the
 spread in velocities of the individual galaxies in the cluster.
 To his surprise, this second <firstterm>dynamical mass</firstterm>
 estimate was <emphasis>400 times</emphasis> larger than the estimate
 based on the galaxy light.
 </para>
 
 <para>
 Although the evidence was strong at Zwicky's time, it was not until
 the 1970s that scientists began to explore this discrepancy
 comprehensively.  It was at this time that the existence of Dark
 Matter began to be taken seriously.  The existence of such matter
 would not only resolve the mass deficit in galaxy clusters; it
 would also have far more reaching consequences for the evolution and
 fate of the universe itself.
 </para>
 
 <para>
 Another phenomenon that suggested the need for dark matter is the
 rotational curves of <firstterm>Spiral Galaxies</firstterm>.  Spiral Galaxies
 contain a large population of stars that orbit the Galactic center on
 nearly circular orbits, much like planets orbit a star.  Like
 planetary orbits, stars with larger galactic orbits are expected to
 have slower orbital speeds (this is just a statement of Kepler's 3rd Law).
 Actually, Kepler's 3rd Law only applies to stars near the perimeter of a Spiral
 Galaxy, because it assumes the mass enclosed by the orbit to be
 constant.
 </para>
 
 <para>
 However, astronomers have made observations of the orbital speeds of
 stars in the outer parts of a large number of spiral galaxies, and
 none of them follow Kepler's 3rd Law as expected.  Instead of falling
 off at larger radii, the orbital speeds remain remarkably constant.
 The implication is that the mass enclosed by larger-radius orbits
 increases, even for stars that are apparently near the edge of the
 galaxy.  While they are near the edge of the luminous part of the
 galaxy, the galaxy has a mass profile that apparently continues well
 beyond the regions occupied by stars.
 </para>
 
 <para>
 Here is another way to think about it: Consider the stars near the
 perimeter of a spiral galaxy, with typical observed orbital
 velocities of 200 kilometers per second.  If the galaxy consisted of
 only the matter that we can see, these stars would very quickly fly
 off from the galaxy, because their orbital speeds are four times
 larger than the galaxy's escape velocity.  Since galaxies are not seen
 to be spinning apart, there must be mass in the galaxy that we are not
 accounting for when we add up all the parts we can see.
 </para>
 
 <para>Several theories have surfaced in literature to account for the
 missing mass such as <acronym>WIMP</acronym>s (Weakly Interacting
 Massive Particles), <acronym>MACHO</acronym>s (MAssive Compact Halo
 Objects), primordial black holes, massive neutrinos, and others; each
 with their pros and cons.  No single theory has yet been accepted by
 the astronomical community, because we so far lack the means to
 conclusively test one theory against the other.</para>
 
 <tip>
 <para>
 You can see the galaxy clusters that Professor Zwicky studied to
 discover Dark Matter.  Use the &kstars; <guilabel>Find Object</guilabel> window
 (<keycombo action="simul">&Ctrl;<keycap>F</keycap></keycombo>) to
 center on <quote>M 87</quote> to find the Virgo Cluster, and on
 <quote>NGC 4884</quote> to find the Coma Cluster.  You may have to
 zoom in to see the galaxies.  Note that the Virgo Cluster appears to
 be much larger on the sky.  In reality, Coma is the larger cluster;
 it only appears smaller because it is further away.
 </para>
 </tip>
 </sect1>