Throughout history, humans have tried to explain how everything came to be. The theories we came up with were conjecture based on intuition and imagination.
Many origin myths describe a duality of light and dark, where a sudden dramatic event brings light and life into a void. Other myths focus on an unending universe that cycles between periods of life and death. Most view the Earth as central to the universe.
The Big Question
Until the twentieth century, scientists felt that the origin of the universe was largely unknowable. They left explanations to the theologians and philosophers. But as telescopes were built that could reach deep into the universe, the curiosity of physicists was irrevocably sparked. They could now see vast structures of galactic clusters stringing throughout the universe. Each cluster was made of many galaxies, and each galaxy was made of millions of stars. How could the Earth be the focus of something so incredibly big? It was just one world among what was probably many (later, we found evidence for extra-solar planets).
The complexity and similarity in galactic structures, and the sheer vastness of the visible universe left nagging questions that weren't answered by the philosophers. First of all, the universe appeared to be very old, much older than the Earth.
Measuring the Age of the Universe
Physicists knew that the speed of light was constant. They also knew how far they could see through telescopes. They determined the distance to other stars and galaxies by using parallax and the relative brightness of special stars called Cepheids. Since light takes time to travel from the far corners of the visible universe, they could estimate how old the universe was by multiplying the distance across the universe by the speed of light.
The universe was so old that most physicists assumed that the universe had always existed. This was called the static universe. This was amended later as it was discovered that the universe was expanding, and the new theory was called "steady state". Throughout the latter half of the century, another theory was beginning to emerge.
The Big Bang
The Big Bang theory proposed that the current universe was the result of an incredible explosion from a single point in spacetime billions of years ago. This cast the universe in a very different light. It was dynamic, evolving, and volatile. Recently there has been wrangling over the details, but there is no evidence that the universe came into being in anything other than a dramatic event.
