Dark Matter


Girl in bubble walker

I can't believe it!

Dark matter was initially called "missing matter" because astronomers could not find it by observing the universe in any part of the electromagnetic spectrum.

There is no current problem of greater importance to cosmology than that of dark matter. Dark matter is composed of particles that do not absorb, reflect, or emit light, so they cannot be detected by observing electromagnetic radiation. Dark matter is material that cannot be seen directly. We know that dark matter exists because of the effect it has on objects that we can observe directly.

Scientists study dark matter by looking at the effects it has on visible objects. Scientists believe that dark matter may account for the unexplained motions of stars within galaxies. Computers play an important role in the search for dark matter data. They allow scientists to create models which predict galaxy behavior. Satellites are also being used to gather dark matter data. In 1997, a Hubble Space Telescope image (seen on the right) revealed light from a distant galaxy cluster being bent by another cluster in the foreground of the image. Based on the way the light was bent, scientists estimated the mass of the foreground cluster to be 250 times greater than the visible matter in the cluster. Scientists believe that dark matter in the cluster accounts for the unexplained mass.

HSt image of a gravitational lens created by a galaxy
Gravitational Lens Created by Galaxy Cluster Reveals Presence of Dark Matter

Scientists have produced many theories about what exactly dark matter may be. Some believe that it may be normal objects such as cold gasses, dark galaxies, or massive compact halo objects (called MACHOs, they would include black holes and brown dwarfs). Other scientists believe that dark matter may be composed of strange particles which were created in the very early universe. Such particles may include axions, weakly interacting massive particles (called WIMPs), or neutrinos.

Understanding dark matter is important to understanding the size, shape and future of the universe. The amount of dark matter in the universe will determine if the universe is open (continues to expand), closed (expands to a point and then collapses) or flat (expands and then stops when it reaches equilibrium). Understanding dark matter will also aid in definitively explaining the formation and evolution of galaxies and clusters. As a galaxy spins it should be torn apart. This does not happen, so something is holding the galaxy together. The something is gravity; the amount of gravity required to do this, however, is enormous and could not be generated by the visible matter in the galaxy.

A Question

If dark matter cannot be seen, how do we know that there is the possibility that it exists?

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The Answer
The Answer


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