If I may interject as somebody who has actually put considerable study into simply the practical aspects of this problem, in response to the first writeup in this node:
Supposing there are 'infinite' universes (a universe for every possible movement of every possible quantum in every possible location in time and space), that would mean that not only are there inevitably bound to be universes containing entities who have the technology to travel to other universes, but there are inevitably bound to be very very many universes that are virtually identical to each other.
This means that any given universe-traveller would not have to visit every single 'version' of one universe in order to have studied or exploited it.
Think of it in terms of Borges' "Library of Babel". In the short story, the library contains a vast number of books with the same number of letters on every page, and the same number of pages. The number of books is ridiculously vast, but it is not truly infinite, because there is only one of each precisely-oriented series of letters and pages. Even so, there could potentially be literally billions of books which are identical except for a single letter out of place. There could be hundreds of billions of books identical but for a single word different. If you had a billion copies of "A Wrinkle in Time", even if you changed one letter or one whole word, it would still read to you as the same story contained in any other copy of "A Wrinkle in Time".
Now apply this to any visitors with advanced, universe-travelling technologies. Suppose their mission is exploratory. They will already know that in a given 'sector' of every possible reality, there will be a vast number of nearly-identical universes. They only have to pick one to research to know the gist of all the rest which bear a strong similarity to it.
In other words, for all we know, we've already had visitors, or another version of this universe has already had visitors, and we simply weren't in the place and time to notice them being here/there. They may simply have gotten all their data from a nearly-identical universe.
In the case of exploitative missions, if you have virtually unlimited realities to exploit, the chance of finding any single, specific reality is effectively so low as to be probability-zero.
After some /msg discussion with Pandeism Fish, I decided it would be suitable to add these points to the writeup.
First Pandeism Fish suggested a Universe destroying Universes, to which I replied "If you destroy three hundred trillion universes, it still doesn't go anywhere near infinity."
Pandeism Fish suggested a Universe destroying ALL other Universes, and I replied "If such a way exists, then it would cease to work immediately, because the 'one' that remains in existence would, at Second 1 following the destruction of the last of the other universes, itself create infinitely more universes with the infinite minute variations in movement of quanta. It's a self-defeating proposition."
Pandeism Fish suggested a Universe perpetually destroying other Universes, and I replied: "if ANY universe had found a way to destroy ALL the others but itself... then you and I could not possibly be having this conversation."