Astronomers use light years to measure distances in space because space is so massive and distances so far that conventional numbering is inadequate and unmanageable. One light year is a distance measurement equivalent to six trillion miles.
The speed of light in a vacuum is a consistent, reliable measurement throughout the universe; it travels at approximately 670 million miles per hour. A light year is a measurement of the distance that light travels in the course of one year. The entire Milky Way galaxy measures approximately 100,000 light years across, and the closest neighboring galaxy, the Andromeda galaxy, is two million light years away.