Nebula’s are the most beautiful astronomical objects in the universe. Nebula’s are normally formed by gathering ionized gases and dust around large area of interstellar medium. Back in early days we use to call nebula for most of the diffusive astronomical objects. Sometime people refer Andromeda Galaxy as Andromeda nebula as well.
Most Nebula’s are very huge. Sometime it spread across hundreds of thousands of Light years. The gravitational interaction among these medium will keep these Nebula together. Sometime those could be the early stage of formation of a star and planetary systems. Or some nebulas are leftover dust and air particles from exploded supernovas.
This indicates that outer space between the galaxies will always not be empty. There are so much of wonders among them. Watch my above video for more details.