Einstein’s theory of general relativity indicated that the universe is expanding. Later observations by Hubble proved this to be the case. Current mathematics now indicates that there may be parallel universes to ours - perhaps an infinite number of them.
It is believed that gravity may be able to cross from one universe to another.
One possible way of demonstrating that parallel universes exist would be to see if there were an increase or decrease of gravity caused by an exchange of gravity between universes.
One of the experiments that will be carried out at the Large Hadron Collider in Switzerland will be to see if particles of gravity (gravitons) can be detected and to observe any increase or decrease in their quantity which would indicate a transfer of gravity to and from other universes.
If parallel universes exist and there are an infinite number of them then somewhere there are identical occurrences of each and every one of us. As an individual I am doing exactly what a duplicate me is doing in a matching parallel universe.
This may sound incredible but much of what we accept now would have been thought implausible, ridiculous, even heretical two or three hundred years ago.