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Cosmology - Dark Matter

More than 90% of the Universe contains material of unknown composition. We know this Dark Matter exists, and the only way we can identify it is by its affect on known objects.

The first clue to the existence of Dark Matter is studying the rotation curve of our Milky Way galaxy. Kepler's laws or rotation (which hold well for planetary orbits around our Sun) states that stars in the outer spiral arm should rotate much slower than stars toward the center of the galaxy; but they don't:

Other evidence that Dark Matter is a real phenomenon:

  • The halo of a galaxy contain more mass than can be seen visually - determined by gravitation effects on the host galaxy
  • Hot X-ray gas found in clusters have remained due to unseen gravitational forces
  • Distortion effects of distant quasars - the gravity lens
  • Light from distant galaxies demonstrate hydrogen absorption lines from unseen matter
  • Motions of galaxy clusters themselves indicate some strong gravity influence from unseen sources

I wrote a project paper on Dark Matter found in the advanced topics section that goes into so detail about determining possible sources of Dark Matter. Basically the two competing theories are:

  • Hot Dark Matter
  • Cold Dark Matter

Black holes, brown dwarfs, white dwarfs and other massive objects only equal a small percentage of unseen matter. Hot dark matter is thought to be near zero-mass moving near the speed of light. This can be relativistic moving massive neutrinos. Cold dark matter compose of more massive particles moving slower than the speed of light.

One interesting effect of Dark Matter surrounding a galaxy is that objects that lay behind the galaxy in the line of sight of an observer will witness what is called a gravity lens. The mass of the galaxy itself is not enough to affect the light from the distant object, but Dark Matter can.

               (Image credit: Brooks/Cole Thomson Learning)

A dramatic lens effect is seen with the nearby cluster of galaxies - Abell 2218:


Computer simulations are very important in determining the role of Dark Matter and the formation of the structure of our Universe. As discussed earlier, galaxies exist in clusters and clusters are members of superclusters. All of this is held together by Dark Matter.

When astronomers use computers to map Dark Matter, the result is something like this image on the left.

This particular frame is a simulation of both Cold and Hot Dark Matter. The stringy material in the simulation represents the distribution of galaxy superclusters. This is in agreement with what we observer.

For a simulation of Dark Matter, click the image on the right. This video shows the gradual formation of structure as time progresses.

This particular video shows a simulation of Cold Dark Matter (video care of Swinburne Astronomy Online).

The problem that astronomers face now is attempting to determine if Dark Matter is Cold, Hot or both. There are two major theories as to the structure of our Universe, and each depends on either Hot or Cold Dark Matter.

  • The Top Down model - Hot Dark Matter dominated Universe break apart to form clusters of galaxies
  • The Bottom Up model - A Cold Dark Matter dominated Universe starts out with small clumps that coalesce into larger clusters of galaxies

Data is still being collected and debates are sometimes very heated.

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