Humans have observed the sky and stars with fascination for thousands of years. In the last century, scientists have began to understand that gravity, the force that pulls objects to Earth, also works over the vast distances of space and affects everything in the universe, including not only matter, but also light. The Swiss astronomer Fritz Zwicky, while studying an area of space called the Coma cluster, found that gravity was not working the way it was predicted to work.
The Coma Cluster (Abell 1656) is a large cluster of galaxies that contains over
1,000 galaxies. It is located in and takes its name from the constellation Coma Berenices.
The cluster should have dispersed long ago, unless it contained more mass than was visible. He called the unseen matter "dark matter". At first other astronomers did not take Zwicky's idea seriously. But as more areas of space were found to not make sense unless there was a large amount of invisible matter present, scientists realized that the universe held a huge amount of dark matter.
In order for Einstein's Equations to work, he had to add a factor that he called the cosmological constant. Today, scientists recognise that the requirement for the cosmological constant is the result of a force called "dark energy" which permeates thoughout space working against the collapse, pushing objects apart as gravity pulls them together.
Dark matter is strange in that it doesn't release or absorb light, or interact with normal matter except through gravity. Scientists so far calculate that visible matter makes up only 5% of the universe. Dark matter and dark energy are make up the other 27% and 68%, respectively.
The universe, has been expanding since it began 13.8 billion years ago. The space between objects is increasing, as if space itself is being stretched apart like the surface of a balloon as it's inflated. Scientists expected that the speed of this expansion should slow down over time, but instead, they've discovered that the expansion of the universe is speeding up. Scientists feel that dark energy is what is causing this accelerated expansion.
Resource: U.S. Department of Energy's Argonne National Laboratory.
