Featured within the musical episode Lexx 2.18 "Brigadoom", the Theatre or the Traveling Troope are a group of beings not native to either the Dark or Light Universes. They are separate from any universe they visit, and immune to anything that might do them harm when they occupy a universe for their show.
The theatre appears after the Lexx passed through a mist which was encountered as they traveled towards the center of the Light Universe. It is possible that their presence is directly linked to the uncertainty of this point in space. Due to their familiarity, it is plausible to suggest that they are amongst the few characters that actually know of and transcend the cycles of time.
Their purpose is to travel various universes (possibly extending beyond the two featured in Lexx) to tell the greatest tales of the universe when its time comes to an end. At the end of their show they will offer those that have played a part in that performance the opportunity to join them. It is possible other members of their Troope joined them in this way.