It's a zoo out there -- a zoo of particles. There are the familiar protons, neutrons, and electrons, which make up atoms. And there are also lesser-known particles like muons and neutrinos, and particles of energy, known as photons.
But the 800-pound gorilla of the particle zoo hasn't yet been discovered: the particle that comprises dark matter.
The motions of stars and galaxies show that most of the matter in the universe is dark -- it produces no detectable energy, but it exerts a gravitational pull on the visible matter around it. Dark matter makes up about five-sixths of all the matter in the universe. And scientists have concluded that it's probably in the form of elementary particles that were created in the Big Bang.
GEBHARDT: Unfortunately, we haven't detected this particle yet. That's why we call it "dark." In science, whenever we don't understand something -- dark matter, dark energy -- we tend to call it "dark."
That's Texas astronomer Karl Gebhardt, who's looking for evidence of dark matter. Pearl Sandick, a physicist with the Texas Cosmology Center, explains a leading candidate for the "dark" particle:
SANDICK: It's probably something we call a WIMP -- a weakly interacting massive particle -- and here we're talking about particles that are a hundred times as massive as a proton.
As the name says, these particles rarely interact with either normal matter or each other, which makes them tough to find. More about that tomorrow.
Script by Damond Benningfield, Copyright 2010
Production and distribution of this week's programs is made possible in part by the Texas Cosmology Center.
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