The universe was born at a specific time in the past, and it has been expanding ever since.
The American astronomer Edwin Hubble discovered two important facts about the universe. First, he showed that matter is clumped together into large collections of stars called galaxies, and, second, he showed that galaxies are moving apart from each other. If you think of “running the film backward,” you see that the Hubble expansion implies that the universe began at a specific time in the past—a little more than 14 billion years ago, in fact. The idea that the universe began in a hot, dense state and has been expanding and cooling ever since is the main tenet of the Big Bang picture of the universe.
We now know that visible matter is actually only a small part of what the universe contains. Over 90% of the mass of a galaxy is made of dark matter. We do not yet know what it is, but we can see its gravitational effect of stars. Furthermore, we now know that the expansion of the universe is accelerating rather than slowing down. This means that there is another kind of material in the universe, dubbed dark energy, that is capable of exerting a force that overcomes the attraction of gravity. Understanding dark matter and dark energy are the primary goals of cosmology.
Today, scientists can trace the history of the universe back to within a tiny fraction of a second of the beginning, both through experiment and theory. Understanding the beginning of the universe is another goal of cosmology.