Answer to Question 1
As the spiral-density wave moves through the gas and dust clouds, it prompts the rapid formation of the massive blue stars we find in the associations, born from hot, pink emission nebulae. But these massive stars evolve very quickly, blowing up as supernovae when hardly out of the nursery; their supernova shock waves reshock the medium, leading to formation of many more less massive Population I stars like our own Sun farther behind the density wave.
Answer to Question 2
No, normal matter adds up to only about 1-3 of that to reach the critical density. Dark matter is perhaps 10 times more abundant, but it still adds up to only 20-30 of the mass needed for closure.
European wildcat feeding its kittens, as illustrated in Brehm's Life of Animals
Nitrogen Cycle
Nursing Student life
U.S. Life Expectancy by Year of Birth
Replication Cycle of H I V