Gabe Perez-Giz (NYU) gave a blackboard talk at lunch about constructing a stable, bound swarm of photons. Yes, photons! It is the relativistic generalization of the search for potential-density pairs for gravitationally bound objects: Can you find a radial mass distribution that is made up of orbits that are themselves orbits in the potential generated by that mass distribution? The audience was skeptical that any stable solutions could exist, but divided on whether any solutions at all exist. I am with Perez-Giz that it is likely that some solutions do exist. Stability, that's another matter.