I believe evidence of other Big Bangs (AKA Other Universes) can be found within our Big Bang (AKA Our Universe). (I'll use BB for Big Bang in this post). As we know, our BB is expanding and the expansion is accelerating. Likewise all the other BBs are expanding the same way and some of their Galaxies will have entered our BB. As we know all our Galaxies are moving away from the place at the center of our BB where the original explosion occurred. The Galaxies that enter our BB from other BBs will be moving in different directions than all our Galaxies. Some would pass through our BB and some would be captured by the massive Black Hole created by the original explosion of our BB. These Galaxies would add to the mass of this Black Hole and eventually expand it until it explodes, creating another Big Bang. This is a simple description of a very complex process.
It is my belief that the only thing that can create a Big Bang is it's interaction with other Big Bangs.
The Galaxies that enter our BB have come a long distance and have been off by themselves for a very long time. They could have collapsed under their own gravity and therefore would not be seen in a telescope but could certainly be detected as massive Black Holes. Finding them would prove the existence of the other BBs.
First we found other stars. Then we discovered our own Galaxie. Then we discovered a couple hundred billion other Galaxies. I believe we will soon discover an infinite number of other Big Bangs AKA Universes.
