Dark Matter
When the Universe was young, it was nearly smooth and featureless. As it
grew older and developed, it became organized. We know that our solar
system is organized into planets (including the Earth!) orbiting
around the Sun. On a scale much larger than the solar system (about 100
million times larger!),
stars collect
themselves into galaxies. Our Sun is
an average star in an average galaxy called the
Milky Way. The Milky Way contains about 100 billion stars. Yes, that's
100,000,000,000 stars! On still larger scales, individual galaxies are
concentrated into groups, or what astronomers call
clusters of
galaxies.
An overlay of an optical image of a cluster of
galaxies
with an x-ray image of hot gas lying within the
cluster
The cluster includes the galaxies and any material which is in the space
between the galaxies. The force, or glue, that holds the cluster together is
gravity -- the mutual attraction of everything in the Universe
for everything else. The space between galaxies in clusters is filled
with a hot gas. In fact, the gas is so hot (tens of millions of degrees!)
that it shines in
X-rays instead of
visible light. In the
image above, the hot X-ray gas (shown in pink) lying between the
galaxies is superimposed on an
an optical picture of the cluster of galaxies.
By studying the
distribution and temperature of the hot gas we can measure how much it is
being squeezed by the force of gravity from all the material in the cluster.
This allows scientists to determine how much total material
(matter)
there is in that part of space.
Remarkably, it turns out there is five times more material in clusters of
galaxies than we would expect from the galaxies and hot gas we can see.
Most of the stuff in clusters of galaxies is invisible and, since these are the
largest structures in the Universe held together by gravity, scientists then
conclude that most of the matter in the entire Universe is invisible. This
invisible stuff is called 'dark matter'. There is currently much ongoing
research by scientists attempting to discover exactly what this dark matter is,
how much there is, and what effect it may have on the future of the Universe as
a whole.
|
