The Universe is everything we can touch, feel, sense, measure or detect. It includes living things, planets, stars, galaxies, dust clouds, light, and even time. Before the birth of the Universe, time, space, and matter did not exist.

The Universe contains billions of galaxies, each containing millions or billions of stars. The space between the stars and galaxies is largely empty. However, even places far from stars and planets contain scattered particles of dust or a few hydrogen atoms per cubic centimeter. Space is also filled with radiation (e.g. light and heat), magnetic fields, and high energy particles (e.g. cosmic rays).

The Universe is incredibly huge. It would take a modern jet fighter more than a million years to reach the nearest star to the Sun. Traveling at the speed of light (300,000 km per second), it would take 100,000 years to cross our Milky Way galaxy alone.

No one knows the exact size of the Universe, because we cannot see the edge – if there is one. All we do know is that the visible Universe is at least 93 billion light years across. (A light year is the distance light travels in one year – about 9 trillion km.)

The Universe has not always been the same size. Scientists believe it began in a Big Bang, which took place nearly 14 billion years ago. Since then, the Universe has been expanding outward at a very high speed. So the area of space we now see is billions of times bigger than it was when the Universe was very young. The galaxies are also moving further apart as the space between them expands.

A few years after Albert Einstein had developed his theory of General Relativity (GR) in 1915 he applied it to the entire universe and found something remarkable. The theory predicts that the whole universe is either expanding or contracting. In 1929 the astronomer Edwin Hubble measured the velocities of a large selection of galaxies. He expected that about equal numbers would be moving toward and away from us. After all, the Earth isn't a particularly special place in the universe. Instead, he discovered that almost all galaxies are moving away from us.

Since the time of Hubble, we have observed millions of galaxies with better equipment and verified his results. Except for a small handful of galaxies close to us, every galaxy is moving away from us. And in fact, the farther away a galaxy is the faster it is moving away from us. This fits in very well with Einstein's predictions. The galaxies seem to be receding from us because the entire universe is getting larger. The space between the galaxies is stretching. And the farther away a galaxy is the more space there is to stretch so the faster the galaxy appears to move away from us.

Over the past half-century, astronomers have observed many other facts about the universe that all point to the fact that the universe is expanding. While a very inventive person might be able to explain away one or at most two of these discoveries, the expansion of the universe is the only theory that can explain all of them at once. And with each passing year, the evidence piles up higher!