Hello, a tidbit from http://skyserver.fnal.gov/en/astro/galaxies/galaxies.asp

Spiral galaxies like the one to the left have flat disks of stars with bright bulges called nuclei in their centers. Spiral arms wrap around these bulges. An extended spherical halo of stars envelops the nuclei and arms. Spiral arms probably form as the result of waves that sweep through the galactic disk. Like the waves on the ocean, these so-called "density waves" don't carry any material with them - they move by disrupting the material they pass through. In the case of galaxies, density waves squeeze clouds of interstellar gas, causing new stars to form inside the clouds. Some newborn stars are massive, hot, and bright, so they make the spiral arms appear bright. These massive stars are blue or white, so the spiral arms look blue-white, too. When viewed edge-on, the spiral arms often appear as dark lanes, because they contain lots of interstellar dust that blocks the light from the bulge. The gaps between the arms contain older stars, which are not as bright. However, the bulges of spirals are often red, like elliptical galaxies, suggesting that they are composed of older stars.

In some spirals, the density wave organizes the stars in the center into a bar. The arms of barred spiral galaxies spiral outward from the ends of the bar. The Milky Way may fall into this class of spirals, called barred spirals.
No idea on your last two questions but I can say, think big. No Bigger. The universe is extremely massive and we are viewing it from only one small point in the milky way so really there is no way (without many perspectives) to locate the "center" of the universe.
On a side note you can post this type of question on the "Space" portion of this board.
Happy Holidays, VINCE