Finding a door to a parallel universe.

If there were a portal linking us to a parallel universe or some other region of space, how would we spot it? One suggestion is that it will give itself away by the curious way it bends light.
The existence of wormholes linking different regions of space was suggested in 1916 by the Austrian physicist Ludwig Flamm as a possible solution to equations of general relativity, which Einstein had published that year. They have since become accepted as a natural consequence of general relativity, which predicts that matter entering one end of a wormhole would instantly emerge somewhere else, so long as the wormhole is somehow propped open.
Though no direct evidence for wormholes has been observed, this could be because they are disguised as black holes. Now Alexander Shatskiy of the Lebedev Physical Institute in Moscow, Russia, is suggesting a possible way to tell the two kinds of object apart. His idea assumes the existence of a bizarre substance called “phantom matter”, which has been proposed to explain how wormholes might stay open. Phantom matter has negative energy and negative mass, so it creates a repulsive effect that prevents the wormhole closing.