The ROYGBIV Theorem states that all rainbows are in fact bridges spanning across the multiverse (See also; Bifrost). The Theorem goes on to proclaim that these "rainbow bridges" were in fact created by Leprechauns in their attempt to collect and protect vast amounts of gold. They use advanced technology to shield the bridges from conventional space-time by making the bridges constantly moving through time at a consistent rate, but move through space at the speed of light. Thus making it impossible to reach a rainbow without proper Leprechaun-ish technology. It also says that if anyone were to achieve light-speed travel, then defense mechanisms in the bridge's systems would cause both ends of the rainbow to deviate wildly from its original path, which gives off the impression that the ends of a rainbow are invisible when in fact they are simply going in another direction. This means that if the ends of the rainbow moves then all the treasure stored at the ends will move too.
This theory was devised by Domovoi The Third and the shadowy being, of fibrous origin: Professor Tweed. It has been thoroughly denied by Coscar O'Herian and Shillelagh O'Rainbow, both upstanding members and prominent figures in the Leprechaun-Fungologian community.