Nearly all the matter in the universe is concentrated in galaxies. A galaxy is a gigantic mass of stars held together by gravity. The largest contain millions of stars. The smallest have just a few million, but even small galaxies are so big that light takes thousands of years to cross them. Despite having so much matter, galaxies are mostly empty space, with vast distances between each star. Our sun and all the stars we can see with the naked eye belong to just one galaxy - the Milky Way. Beyond this lie billions of galaxies stretching as far into space as astronomers can see.
Nearly all the matter in the universe is concentrated in galaxies. A galaxy is a gigantic mass of stars held together by gravity. The largest contain millions of stars. The smallest have just a few million, but even small galaxies are so big that light takes thousands of years to cross them. Despite having so much matter, galaxies are mostly empty space, with vast distances between each star. Our sun and all the stars we can see with the naked eye belong to just one galaxy - the Milky Way. Beyond this lie billions of galaxies stretching as far into space as astronomers can see.