Imagine if you take two complete universes. You set one on a table, and then you set the other right down in the same place. Because of the distances between suns and other stellar objects, many stars and even galaxies of the second universe will simply occupy empty space in the first universe. The dispersion of matter in our own universe leaves a heck of a lot of empty space. But because these two universes occupy the same place but not the same space, it's irrelevant in most all cases. You cannot in any way detect either universe from the other.
But the odds are that some few stars will occupy the exact same places in both universes.These two universes don't interact normally, but placing two gravity wells in the exact same place could possibly cause the two universes to interact at those small locations. It is an idea that has no basis in fact that we know of, but that doesn't mean it's impossible.
Only 100 years ago, it was thought that the entire universe consisted of the Milky Way galaxy alone. The discovery of Cepheid variable stars, which vary in luminosity at a predictable rate everywhere in the cosmos, allowed scientists to locate their physical position in space by comparing their apparent luminosity to their intrinsic luminosity to give their distance from Earth.
At that time, the distant blotches of stars seen in the heavens were thought to be nebulae of dust within our own galaxy. Edwin Hubble located a Cepheid variable in one of these nebulae, and determined its distance to be so far from the earth and the Milky Way that it had to be another galaxy beyond our own. So it was only a century ago that the size of the universe was totally reassessed, and discovered to be filled with other galaxies than our own.
The science we know today that seems like fact is subject to change tomorrow. Science is one field where we can never safely say never!