According to Universe Today, light travels approximately 9.5 trillion kilometers or 5.9 trillion miles in a year. Light years are commonly referenced units, but astronomers tend to use a unit known as the parsec, equivalent roughly to 3.26 light years.
The remarkable length of light years goes some way towards explaining why humans have not traveled far into the cosmos. The Alpha Centauri star system is around four light years away, while the Canis Major Dwarf Galaxy, the nearest galaxy to Earth, is around 25,000 light years away. As of 2014, travel at the speed of light, which is 3 x 108 m/s, is deemed impossible for any physical object.