Dark Experiments 
To find the most common particles of matter in the universe, physicists are going to extremes. They’re putting detectors both high above Earth’s surface and deep below it. These instruments could see traces of perhaps a handful of particles a year — even though millions of them may stream through the instruments every second.
The instruments are looking for dark matter. It accounts for more than 80 percent of all the matter in the universe. Yet no one has ever seen even a single particle of it because it produces no detectable energy. Scientists know it exists only because it exerts a gravitational pull on the visible matter around it.
Dark matter likely consists of some type of subatomic particle. These particles almost never interact with normal matter, though, making them extremely difficult to find. But there are so many dark—matter particles that there should be a few collisions that produce some sort of reaction.
Experiments in space are looking for the aftermath of these collisions at certain wavelengths of light. And experiments on Earth look for the collisions themselves in detectors placed deep underground. Most normal particles are blocked by the rock above the detectors, but dark matter streams through. A collision between a dark matter particle and a normal particle should produce a detectable reaction. But such collisions are so rare that no experiment should see more than two or three of them in an entire year.
More about the search for dark matter tomorrow.
Script by Damond Benningfield, Copyright 2013