Careful study of the galaxy’s rotation reveals one of the greatest mysteries in science. Stars at different distances from the galactic center orbit at different speeds, and we can learn how mass is distributed in the galaxy by measuring these speeds. Such studies indicate that the stars in the disk of the galaxy represent only the ”tip of the iceberg” compared to the mass of the entire galaxy. Most of the mass of the galaxy seems to be located outside the visible disk, in what we call the halo. We do not know the nature of this mass, but we call it dark matter because we have not detected any light coming from it. Studies of other galaxies suggest that they are also made mostly of dark matter, which means this mysterious matter must significantly outweigh the ordinary matter that makes up planets and stars. An even more mysterious dark energy seems to make up much of the total energy content of the universe.