Up until now, the theory of multiverses was thought to be untestable. But they might have just come up with something:
"If the universe that we inhabit had long ago collided with another universe, the crash would have left an imprint on the cosmic microwave background (CMB), the faint afterglow from the Big Bang. And if physicists could detect such a signature, it would provide a window into the multiverse ...
In the years since their initial meeting, Peiris and Johnson have studied how a collision with another universe in the earliest moments of time would have sent something similar to a shock wave across our universe. They think they may be able to find evidence of such a collision in data from the Planck space telescope, which maps the CMB.
The project might not work, Peiris concedes. It requires not only that we live in a multiverse but also that our universe collided with another in our primal cosmic history. But if physicists succeed, they will have the first improbable evidence of a cosmos beyond our own."
Many discoveries in the universe were initially made by inference not observation, such as black holes and dark matter.