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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
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