The mass of the Galaxy Can be determined with a measurement of the rotation curve. Comprised mostly of dark matter. Top down vs. bottom up theories of galaxy formation. Spiral Arms Contain young stars and gas clouds. Explained by spiral density wave theory.
Density waves trigger star formation.
Rapid orbital motion of stars near the Galactic center and a radio jet support this conclusion. 100 times a galaxy host galaxies seen by Hubble Space Telescope a few quasars are gravitationally lensed. The fate on the Universe depends on its geometry. Universe has positive curvature and will recollapse. Universe had negative curvature and will expand forever.