Most of the matter in the universe is hiding in plain sight. It surrounds galaxies, including our own Milky Way. And its gravity binds together clusters of galaxies. But it produces no detectable energy, so the only way we know it's there is through its tug on the visible matter around it.
That makes it hard to identify just what this "dark matter" is made of. Scientists think it probably consists of subatomic particles, but they've never seen one. And the particles seldom interact with either normal matter or other particles of dark matter, so it's hard to catch one.
Still, that doesn't stop scientists from trying, as Texas physicist Steven Weinberg explains:
WEINBERG: One approach, the most direct, is just to wait for them to hit us. That is, take large tanks of detector material. Most of the dark matter particles will just go right through, but occasionally one will hit a nucleus in the detector material and cause it to recoil. These experiments are going on in various underground laboratories throughout the world.
A second technique is to watch for dark-matter particles to ram together. Although such collisions are rare, there are so many particles in the universe that a few of them are bound to come together. When they do, they should annihilate each other in a burst of energy and normal particles. A space telescope is looking for such outbursts.
But the best way to find a dark-matter particle may be to make one. More about that tomorrow.
Script by Damond Benningfield, Copyright 2010