Space Science Short


National Aeronautics and
Space Administration

NASA Headquarters
Washington, D.C.                                              November 1994
______________________________________________________________


                                DARK MATTER

All visible celestial objects known today account for only 10% of the mass
in the universe. The rest of this "missing mass," also known as "dark
matter," is presumably invisible because it does not emit or reflect visible
light or other forms of electromagnetic radiation.  Or perhaps its light is so
feeble that current astronomical instruments are unable to detect it. 
However, dark matter can be indirectly detected due to its gravitational
influence on other nearby visible objects.

The presence of dark matter was first discovered in 1932 by astronomer
Jan Oort, who measured the perpendicular motions of nearby stars relative
to the disk of our Milky Way. He studied the gravitational influence of the
galactic disk on these stars, and so, was able to measure the mass of the
disk (just as the mass of Earth can be calculated from the acceleration of a
falling object).  To his surprise, this calculated mass was twice the amount
of mass seen as stars and nebulae.  A year later, Fritz Zwicky examined
the dynamics of clusters of galaxies, and also came to the startling
conclusion that the observed galaxies only accounted for 10% of the mass
needed to gravitationally bind the galaxies in the cluster. 

One widely-used method to deduce the amount of missing mass involves
measuring the rotation speed of a spiral galaxy.  Spectroscopic and radio
observations have obtained the rotation velocities of hundreds of spiral
galaxies.  These experiments have revealed that, in most cases, a galaxy's
mass continues to increase toward the edge of its visible disk of stars. This
implies that spiral galaxies are surrounded in haloes of matter that cannot
be seen. Observations of elliptical galaxies, groups, and clusters of galaxies
also indicate the presence of dark matter interacting gravitationally with the
visible objects.

The nature of dark matter, and its abundance, are among the most
important questions in modern cosmology today.  What is it made of? 
Some astronomers believe that dark matter is composed of protons and
neutrons, called baryonic or simply "normal" matter.  Baryonic dark matter
candidates include extra-solar planets, remnants of stellar evolution such as
comets, objects not massive enough to ignite hydrogen fusion called brown
dwarfs, dying embers of stars such as cold white dwarfs and neutron stars,
as well as interstellar and intergalactic gases. 

Non-baryonic dark matter, on the other hand, could be elementary particles
that do not interact strongly with normal matter.  Except for the neutrino
particle, many such elementary particles are still in the realm of theory and
have not been detected. 

Since all visible matter is only a small fraction of the total mass in the
universe, the amount of dark mass that is present will determine the
evolutionary future of the universe.  If there is not enough dark matter to
gravitationally bind the universe together, it could continue expanding
forever.  If there is enough mass in the universe to gravitationally hold it
together, the universe may slow down its expansion, come to a halt, and
begin to contract and eventually collapse. 

The temperature of dark matter in the early universe also may have
determined the early evolution of the universe.  Not long after the Big
Bang and prior to the formation of galaxies, matter began to aggregate
under the influence of gravity.  Dark matter might have provided the
"seeds," a lumpy background in which ordinary matter could congregate to
form galaxies and stars.  If this "cold dark matter" were present, where
particles had a negligible random motion, galaxy formation would begin on
small scales.  Matter would gather in sizes comparable to current galaxies
or smaller, and eventually build to become clusters and superclusters due to
the gravitational attraction of the galaxies. 

If, however, "warm dark matter" was present, it would erase the small
galaxy-sized "seeds" that initially formed.  Instead, enormous gaseous
pancake-like structures as large as superclusters and clusters, are created,
subsequently condensing into individual galaxies.