Our Universe May Be One Of ManyThe Universe is all of space and time (spacetime) and its contents, which includes planets, moons, stars, galaxies, the contents of intergalactic space and all matter and energy. While the size of the entire Universe is still unknown, it is possible to measure the observable universe. The earliest scientific models of the Universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the centre of the Universe. Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the centre of the Solar System. In developing the law of universal gravitation, Sir Isaac Newton built upon Copernicus's work as well as observations by Tycho Brahe and Johannes Kepler's laws of planetary motion. Further observational improvements led to the realization that our Solar System is located in the Milky Way galaxy, which is one of many galaxies in the Universe. It is assumed that galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center. Discoveries in the early 20th century have suggested that the Universe had a beginning and that it is expanding at an increasing rate. Roughly eighty percent of mass in the Universe appears to exist in an unknown form called dark matter which cannot be directly observed.
"The cosmos is all that is or ever was or ever will be. Our feeblest contemplations of the Cosmos stir us— there is a tingling in the spine, a catch in the voice, a faint sensation, as if a distant memory, or falling from a height. We know we are approaching the greatest of mysteries." — Carl Sagan, Astronomer