University of California, San Diego
Center for Astrophysics & Space Sciences

Gene Smith's Astronomy Tutorial

Dark Matter in the Universe

Dark Matter

Observations of clusters and their galaxies, have uncovered one of the major mysteries in astronomy today. Clusters appear to be very stable entities - they contain mature galaxies with old stars, and seem to have been formed billions of years ago. But, when we calculate the amount of mass in a cluster using the orbital motions of its member galaxies, the result is too low for the cluster to be gravitationally bound. If the cluster contains only the mass we can observe, the gravitational force is insufficient to prevent the galaxies from "escaping".There must be more mass in the cluster than what we see.

Rotation Curves for 3 Spiral Galaxies - Galaxy Image(left), Spectrum (center - photographic negative), & Plot (right).
The flatness of the rotation curve with no downward turn indicates that the mass distribution extends far beyond
the measured values, probably in the form of a massive halo of dark matter.

The same problem arises when we look at the galaxies themselves. The rotation curve of a galaxy shows how the orbital velocities of the stars change with distance from the center. If the galaxy rotated as a solid disk, the velocity would increase linearly with distance. If most of the mass were concentrated at the the center, as in our solar system, the velocities of the stars would decrease with the square root of the distance. But, that is not what is observed. Far past the point where no mass is visible, the rotation curves are flat! This means that the mass is still increasing as we move outward, even though we can't see anything! One again we have to call upon "dark matter". The galaxy must extend much farther out than the luminous matter indicates. In fact, the calculations require that there be at least 10 times more mass than we can see! Calculations suggest that this dark matter is likely to be in an extensive halo of dark matter.

The nature of this dark matter or "missing mass" is unknown. There are theories ranging from the bizzare to the mundane, none of which successfully answer all of the questions.

Dark Matter Links

Gravitational Lenses   Clusters   Outreach & Education   CASS Home  

Comments? Gene Smith
Conducted by:
Prof. H. E. (Gene) Smith
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Last updated: 26 April 1999