The Shadows are an ancient race, possibly billions of years old who have evolved into beings of light, but can manifest themselves physically as well in insect-like form.
They believe that chaos and war are the ways to make the other races develop, with weaker races being eliminated.
Shadow technology is organic and incredibly strong, which allowed the Shadows to cause terror throughout the galaxy.
Only another ancient race - the Vorlons managed to keep the Shadows in check, inflicting several defeats on them which forced them to flee and go into hibernation.
Telepathic ability has the potential to disrupt Shadow technology, and this has been used effectively by the other races.