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'Dark Matter' is something of a shock term for physicists. It's not really very complicated or hard to understand, but is useful for making people in general believe that physicists are like the Wizards of old, with some profound knowledge that mere mortals cannot comprehend, giving them mysterious powers.

Dark Matter is exactly what it sounds like. It is matter that does not shine.

Stars shoot out light in every direction: they are fairly easy to see. Other clumps of matter rarely do. Dark Matter is, basically, anything that isn't a star.

Earth is a clump of Dark Matter - it does not shine on its own. So is Jupiter, for that matter. Black Holes, and most free atoms floating in the void are in the club too.

Dark Matter comes in two categories: MACHOs - Massive Astrophysical Compact Halo Object (big things), and WIMPs - Weakly Interacting Massive Particles (small things). Now, the first term, MACHO, is something of a misnomer, used to throw more technical language into a phrase than need be there. MACHOs need not exist in a galactic halo, for one. The chief reason for the name comes from the root of the Dark Matter problem.

In general, galaxies outer rims are rotating faster than most Astronomers think they should. In the typical modern practice, Cosmologists sought out the most improbable reason for the phenomenon they could think of, and arbitrarily declared it to be the factual answer. That answer is more mass along the edge of the galactic rim, or in the 'halo'.

Bear in mind that these are the same people who proposed that galactic spiral arms formed because of a phenomenon similar to traffic congestion.

Now, don't get me wrong: Dark Matter does exist. There are planets, asteroids, and all manner of objects, made of matter that does not shine, inside our very galaxy. There are probably huge amounts of them in the halo also, and in the intergalactic medium.

The other end of the Dark Matter spectrum is real too - there are all sorts of subatomic particles flying about the cosmos.

The thing of it, though, is whether there is 'enough' of it.

In our Solar System, the sun, Sol, is the sole clump of 'non-dark' matter, but it makes up 98.9% of the mass of the entire system. All the Planets, their moons, the asteroids, the comets, and even the little green men in their shiny spaceships, only make up just under 1%. The remainder of mass is composed of single molecule 'floaters' or smaller items, right down to the neutrinos.

Cosmologists predict that the universe has to be filled

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