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What dark matter is not

Most of us have heard of mysterious, so-called dark matter lurking somewhere among the stars in the universe. If asked to describe what dark matter is in 25 words or less, we'd probably fail the test - myself included. Understanding the idea of dark matter involves complex physics and mathematics. So, for you and me, we'll just have to take the word of the experts on faith.

What exactly is the dark matter astrophysicists and astronomers have been abuzz about since the 1980s?

First, our current understanding of the material composition of the cosmos - and this is based on both advanced mathematics, direct observation, and extrapolation - breaks down thus: 70 percent dark energy, 25 percent dark matter, and 5 percent normal matter (the stuff you and I are made of).

As you can see, there's a lot of dark matter out there - a lot more than normal matter. So where is all this stuff? And is any of it on or near the Earth and is it dangerous?

NASA scientist Paul Hertz likes to say, "We are much more certain of what dark matter is not than we are what it is."

Let's go along with Hertz's reasoning and see, first, what dark matter is not. Perhaps by the process of elimination we can come closer to understanding what this stuff really is.

•According to Hertz, dark matter is - well - dark. This means it does not shine of its own (thermonuclear fusion like the Sun) or reflect photons/light (like the Moon). It is not in the familiar form of stellar and quasi-stellar objects we see with our naked eye or through a telescope, such as stars and planets, asteroids, comets, etc.

Hertz noted that Earth- and space-based observations reveal that there is just too little visible stuff (normal matter) in the cosmos, so you can't explain the "missing" 25 percent of matter by simply shrugging your shoulders and saying, "Oops, I guess we can't see all the dark clouds composed of normal matter in space, that is, the normal matter made up of particles called baryons. So this must be what we imagine to be dark matter."

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