Many experiments are also currently operating to detect dark matter in the form of elementary particles. Measurements of the velocities of stars and gas near the Galactic Center imply the presence of a large concentration of mass there. There is also a radio source right at the Galactic Center which is unlike anything that exists anywhere else in our galaxy. Many astronomers believe that these facts can be explained by invoking a black hole of approximately one million solar masses. According to the Hubble Classification Scheme , galaxies come in four basic types.
Spirals have a central bulge with a flattened disk, generally showing spiral arms.
Barred Spirals are the same as spirals except for the presence of a bar in their central regions. They usually have little gas and dust compared to spirals, few young stars, and no spiral structure. Irregulars are everything else. The Milky Way Galaxy is probably a barred spiral. With the exception of a few nearby galaxies, the spectra of all galaxies is redshifted, indicating that the universe as a whole is expanding.