Stars form in a nebula, a giant cloud of gas and dust.  Over millions of years, the gas and dust clumps together to form protostars.  Gravity inside of each protostar works to make the matter denser and when it becomes hot enough, nuclear fusion begins and the star begins to shine.

Dark nebulae are dense areas of gas and dust that block out any light coming from behind. They are often seen in conjunction with emission nebulae, which create their own light from ionized particles, or reflection nebulae that reflect the light of nearby stars. Very large dark nebulae often appear to the naked eye as dark patches against the brighter areas of the Milky Way.