When man began to search the skies, he saw a lot of things. In the universe(宇宙), the earth seemed to be just a small bit of dust. It is only a small planet, and travels around the sun. The sun together with its planets, travels in the Milky Way(银河), our galaxy(星系). The Milky Way has about thirty billion stars like our sun, and is one of about a hundred million galaxies in the universe. Because the space (空间)is so great, it is difficult to work out how far away a star is. Instead of kilometres, we have light-years. A light-year means how far light travels in one year. Our nearest neighbour galaxy is more than 9000,000 light-years away from the earth. Man has known that ______ .