The large dark nebula (NGC 281A that makes up this "gaping mouth" is a molecular cloud made up predominantly of molecular hydrogen, but also dust and other gases. New generations of solar systems are being forged within its cold interior. Once these young stars' fusion engines switch on they will irradiate their surroundings - heating up, ionizing and eroding away the remaining dark material from which they formed.
Originally the whole Pac-Man nebula would have been one large dark molecular cloud - the stars that formed early on at its centre having progressively hollowed out the centre of the nebula. The gas in and around this central region is ionized by the copious UV radiation emitted by the central open star cluster (IC 1490), causing it to glow and providing the light by which this narrowband image was taken.