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