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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 is 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: <citation>It's a fairly embarrassing situation to admit that we
+can't find 90 percent of the universe</citation>.  </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 suprise, 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; Find Object 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>