Dark nebula on Gum nebula. True-colour optical image of a dark nebula silhouetted against the bright Gum nebula in the constellation Vela. Dark nebulae are dense clouds of interstellar gas and dust which absorb the majority of the light which falls on them. They are usually only detectable by observing them against a bright background, as here. The Gum nebula, an enormous cloud of glowing, ionised hydrogen gas, is the largest nebula in the sky, with a diameter of around 60 full Moon widths. This image was produced by digitally combining photographs taken by the UK Schmidt Telescope in blue and red light.