6-Geon
As is best shown in the equation E=MC2, energy and matter are fundamentally connected. One effect of this is that energy, as well as mass, creates a gravitational field.
A geon, first investigated by John Wheeler, in 1955, is an electromagnetic or gravitational wave whose energy creates a gravitational field, which in turn holds the wave itself together in a confined space. Wheeler speculated that there may be a link between microscopic geons and elementary particles, and that they might even be the same thing.
A more extreme example is a “kugelblitz” (German for “ball lightning”), which is where such intense light is concentrated at a particular point that the gravity caused by the light energy becomes strong enough to collapse into a black hole, trapping the light inside. Although nothing is thought to prevent the formation of a kugelblitz, geons are now only believed to be able to form temporarily, as they will inevitably leak energy and collapse. This unfortunately indicates that Wheeler’s initial conjecture was incorrect, but this has not been definitively proven.