The Universe is everything...all matter and space. It is made up of billions of galaxies, which are each composed of billions of stars, which each have planets, comets, and asteroids orbiting around them. Many planets have moons which orbit them. From largest to smallest, the general structure of the Universe is as follows: Universe, galaxies, solar systems, stars, planets, moons. To measure the incredibly large distances in space, astronomers use either astronomical units (the average distance from the Earth to the Sun, approximately 93 million miles) or light years (the distance a beam of light will travel in one year, approximately 5.8 trillion miles).
The Universe is thought to have formed approximately 13.8 billion years ago in a period of rapid expansion known as the Big Bang. We have developed this theory based on the study of light from all over the Universe. We collect different types of light (energy) with telescopes and analyze it to learn more about the object that emitted it. The only difference between the different types of energy is their wavelength.
By studying the light from distance galaxies, we can determine if those galaxies are stationary, approaching, or receding (moving away from us). If the spectrum of the light from the galaxy or star is red-shifted, the galaxy is moving away. If it is blue-shift, it is approaching. If it is not shifted at all, the object is stationary. It has been determined that the vast majority of galaxies in the Universe have severely red-shifted spectra, meaning they are moving away and the Universe is expanding.
Astronomers have also observed a low-level microwave radiation flying in all directions of space. This cosmic background radiation is thought to be the leftover energy from the Big Bang.
In summary, the two pieces of evidence that support the Big Bang Theory are:
1. The Universe is expanding (most galaxies have red-shifted spectra).
2. Cosmic Background Radiation is the echo of the Big Bang.