Our galaxy, the Milky Way, is a spiral galaxy containing stars, gas, dust, and dark matter. It has a disk with spiral arms where new stars are forming, surrounded by a spherical halo of older stars. Measurements of variable stars like Cepheids and RR Lyrae helped map out the true extent of the Milky Way by establishing their distances. The rotation curve of our galaxy suggests over twice as much dark matter as visible matter. There is evidence of a supermassive black hole at the very center of the Milky Way.