Dark Matter Explained

We’ve heard the term tossed around quite a bit. We’ve seen many depictions, accurate and inaccurate of the stuff, but we probably don’t understand it unless we are physicists. To try and explain dark matter, it must be broken up into two parts. First, why does it exist? And second, what is it exactly?

It is our belief that the Universe is balanced between open and closed. The universe is huge and therefore there needs to be a lot of matter to accomplish its size. By studying the Big Bang theory, we can make an estimation of the baryonic matter of the universe. It is accomplished be connecting the observed ratio of helium to hydrogen (He/H) of the universe today to how much baryonic matter was present during the hot phase when the majority of helium was produced. When the temperature of the universe dropped, neutrons began decaying and becoming protons. If early baryon density was low, then protons were hard-pressed to find neutron mates to create helium before they decayed too far to make the amounts of helium we currently see. So, measuring the He/H ratio, we can estimate the baryon density that was needed to exist shortly after the Big Bang. This in turn makes it possible to estimate the total number baryons today. The resulting estimations show that 1/20 of the universe’s mass is baryonic matter. Our best estimates can only include what we can see, so say the mass of the universe is a whole 100%, and we can only see 1% of the stuff that makes up the universe. Well, where then is the rest of the universe?

There are a couple of conclusions. We see only about 0.01 Mass out of 0.05 Mass baryonic matter in the universe so the rest must be in the dark matter halos surrounding galaxies.

But the other conclusion, for those who don’t trust the Big Bang models, is achieved through more direct paths. The movement of clusters of galaxies suggests that they are bound by a gravitational pull that is five to ten times as much matter that can be accounted for in observation of luminous matter. In individual galaxies you can measure the rate of rotation of the stars around the center. This movement as observed would require about five times more mass than can be seen. Because of the extra mass requirements, we can work under the assumption that galaxies have dark matter halos surrounding them.

So, there are many possibilities to what this dark matter actually is. Nobody really knows anything more than just what they theorize. There are many published suggestions including dark galaxies, brown dwarfs, planetary materials (rocks, dust, etc.), neutrinos or exotica.

These can be placed into two categories: those which are solutions to other problems, but just happen to fit the dark matter problem, and those that have been proposed specifically for the problem of dark matter.

So, although we don’t know yet what dark matter actually is, we are working on solving the case and will no doubt be able to explain it someday.

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