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  • Jeet Ahuja

Dark Matter

A theoretical model of the composition of the universe to the combined set of cosmological observations, scientists have come up with the composition which is: ~68% dark energy,~27% dark matter, ~5% normal matter.


What is dark matter?

We are much more certain what dark matter is not than we are what it is. 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. Although dark matter isn’t simply dark: it’s invisible. Light of all types seems to pass through as though it’s completely transparent. However, dark matter does have mass, which we see by its gravitational influence.

Studies of galaxies show stars and gas moving as though there’s a lot more mass than we can see pulling them along. Based on the motion of what we can observe, galactic dark matter resides in a 'halo' surrounding the ordinary matter of the galaxy. Galaxy clusters can contain hundreds or thousands of galaxies, each of which have its own dark matter halo. However, the cluster has its own dark matter, which outweighs everything else put together. This dark matter influences how individual galaxies and hot gas move inside the cluster.



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.


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