1. The scenario for the formation of galaxies is very reminiscent of the formation process for solar systems.
Giant cloud of gas and dust, shrinks under its own gravity, rotation produces a flattened disk.
The halo objects probably formed first and retained their spherical distribution and didn’t get pulled into the disk.

2. Spiral arm structure
The combination of using 21 cm observations and the Doppler shift due to the different line of sight velocities as you look in different directions through the galaxy, allows us to see the spiral structure

3. With this information astronomers can recreate the 3 dimensional structure of our galaxy including the number of spiral arms, etc.
This is what we think our galaxy looks like.

4. Astronomers still can’t explain well why we have spiral arms in galaxies.
The stars, gas, and dust in the galaxy all obey Kepler’s laws of planetary motion. Thus the inner matterial rotates faster than the outer matterial. If spiral arms were tied to the galaxy then after a few 100,000,000 years they would all be wound up and loose their shape.

5. The density wave theory says that a compression front moves through the disk and compresses the gas and dust to start the star formation process. In this model new matterial is being constantly fed into the density regions as old matterial leaves.

6. Since material orbiting the galaxy must obey Kepler’s laws, if we could measure the distance and orbital period of material as a function of distance from the center, we could determine the amount of mass inside that distance.
When we reach the “edge” of the galaxy, we expect the rotation speed to diminish, but instead it keeps on increasing as if there is some “invisible” (dark) matter.

7. One attempt to detect dark matter is to look for gravitational lensing
One attempt to detect dark matter is to look for gravitational lensing. Even if the matter is dark it should disturb the space-time continuum and produce gravity like any other matter.
While we have seen several such events, they could only account for about 1/2 the dark matter.

8. The Galactic Center
While visible light cannot penetrate the gas and dust to see the center, infrared telescopes can.

10. By using Kepler’s 3rd law and determining the period and semi major axis of the orbits of stars around the galactic center, we can deduce the mass inside their orbits. Results indicate 4 million solar masses inside 10 AU. What could it be?