Dark Matter 
In the 1930s, Fritz Zwicky made two remarkable discoveries. The brilliant but cantankerous astronomer found that galaxies tend to group together in clusters, bound by their mutual gravitational pull. In fact, our home galaxy, the Milky Way, is a member of a small cluster known as the Local Group.
But Zwicky also discovered that the clusters should be flying apart. The gravity of all their visible matter -- their stars, gas, and dust -- wasn't strong enough to keep the galaxies from going their own separate ways. Zwicky surmised that something else was holding the clusters together -- a type of matter that produced no light, but that exerted a strong gravitational pull on the visible matter around it.
Other astronomers quickly accepted the idea of galaxy clusters. Over time, in fact, they've found that the biggest clusters consist of thousands of galaxies and span hundreds of millions of light-years.
But for decades, Zwicky's other discovery was ignored. Yet that second discovery was more important to our understanding of the universe. It told us that most of the matter in the universe is different from the normal atoms that make up stars, planets, and people. Today, scientists are searching for that missing matter, which is known as dark matter.
It's a search that's taking them from the infinitesimal to the infinite -- from the building blocks of matter to the farthest reaches of space. More about that search tomorrow.
Script by Damond Benningfield, Copyright 2010