As you know, light travels at an incredible speed: roughly about 300,000 kilometers per second, or 1,8000,000 kilometers per minute, or 1080000000 kilometers per hour (!). A light year, in astronomy, is the distance which light traverses during one standard Earth year. Just calculate how many hours there are in one year and multiply this quantity by 1080000000, and you will receive the distance in kilometers. This will be just one light year. Considering there are stars, galaxies, and other objects located hundreds of thousands light years away from the Earth, you can imagine (or more likely, cannot--because the human mind just cannot imagine such distances) how big our universe is.