Where is the Bifrost?

Excuse me, could you possibly shed some light on the enigmatic location known as the Bifrost? It seems to be a fabled bridge that holds great significance in various mythologies, but I'm having difficulty pinpointing its exact whereabouts. Is it a physical entity that exists in our realm, or is it a metaphorical construct used to symbolize something deeper? And if it does indeed exist, what mysteries might it hold for those brave enough to venture across its majestic span?