'In this image, organic molecules called polycyclic aromatic hydrocarbons are tinted green. These molecules are formed any time carbon-based materials are burned incompletely: for example in the sooty exhaust from car and plane engines.'

Yes, that's how they're formed on Earth, but not in space where there's no oxygen to support combustion.

Instead it's most likely to be ultraviolet light building PAHs up from smaller molecules. A similar process can be seen in the outer solar system where the moons of Saturn and Uranus, originally made of ammonia and methane have been turned almost black by billions of years of UV light creating complex organic compounds.