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The Question
(Submitted July 31, 1997)
I have taken several high energy physics courses in College and know some
astronomy. Isn't Dark Matter theory just a weak attempt to mesh theory
with observation or has there been any research with empirical results
proving the existence of 'Dark Matter?' Any additional references to
information would be helpful.
The Answer
We have a brief explanation on dark matter at:
http://imagine.gsfc.nasa.gov/docs/introduction/dark_matter.html
To supplement:
Astrophysicists have accumulated a large body of evidence for dark matter.
In this context, 'dark matter' means just that --- matter of whatever type
that does not shine brightly (in visual light, X-rays or at any other
wavelengths). Even though we do not see dark matter directly, its
gravitational influence can be seen in the motion of gas and stars in
galaxies, and in the motion of hot gas and galaxies within clusters of
galaxies. There is recent evidence from microlensing observations that
at least some of the dark matter in our own galaxy is in the form of MACHOS,
or MAssive Compact Halo ObjectS --- these are planets or stars, made up
of ordinary (baryonic) matter, that are too faint to be observed directly,
but can act as a gravitational lens and magnify the brightness of brighter
stars in the background. There is nothing 'weak' in the observational proof
for dark matter in this sense.
In a cluster of galaxies, we can estimate the masses of stars in the galaxies
and the hot gas that fill the cluster. We can also infer the total mass of
the cluster that is needed to keep it gravitationally bound. The latter is
typically found to be ~5 times the combined mass of the stars and the hot gas;
an analogy with our Galaxy suggest that only some of the dark matter can be
MACHOS. Although circumstantial, such results point strongly to the presence
of non-baryonic dark matter in the clusters of galaxies.
When it comes to deciding what kind of exotic particles may make up
the non-baryonic dark matter, however, there may be a hint of 'weakness',
in that different particle physicists favor different exotic particles.
Moreover, as far as I know, there has not been a direct detection of these
exotic particles.
Best wishes,
Koji Mukai for
Ask a High-Energy Astronomer
with help from Dr. Mushotzky
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