On a clear night we can see the band of the Milky Way in the sky, the fuzzy light that stretches from one horizon to the other. This band of light is made up of a vast number of stars that telescopes can pick out as individuals but our eyes cannot. This is our view of the billions of stars that make up the disk of our galaxy, from a viewpoint inside the disk, about two- thirds of the way out from the center of the galaxy to the edge of the disk. There are dark patches in the band of light, due to dark clouds of interstellar matter. Radio telescopes can see through these clouds of dust and have shown that the material in our Galaxy is distributed in a disk with spiral arms of material trailing through. Our Galaxy has a bulge of stars at the center, wherein there may lie a giant black hole at the center. The galactic center lies in the direction of the constellation Sagittarius.
The stars in the disk of the galaxy orbit around the center in a way similar to the way in which planets orbit around the Sun. The speed of each star depends on its distance from the center. The Sun moves at about 250 km/s in its orbit, and it takes 225 million years to complete one revolution around the center. Studying the way in which the stars orbit gives details of the gravitational field of the Galaxy and ultimately reveals its mass. This total mass is about 1 trillion times the mass of our Sun, which is about 10 times the mass of all the visible stars in the Milky Way put together. This mass that is unaccounted for is strong evidence that there is dark matter in our galaxy, matter that is affecting the gravitational field of the Galaxy but is not visible to us.