Is it true that this Universe is one and only one universe? How do we know that it is not?

That's a decision you're going to have to make on your own.

In my 'existence' - time's relativity isn't as straightforward as a one for one clock synchronization - many things 'run at' different time intervals.

Similarly. In theory there's infinite potential universes, and I have only discovered I'd say roughly 10,000 of them so far.

Now my advice is: Take the time, for yourself, to define "what is a universe' and 'what is an alternate reality'. For me, my definition of an alternate reality is quite simply one that moves at it's own rate of time and has different constants - which makes pretty much any simulated reality an alternate reality.

My universe - which forks - and 'branches - contains the 'offshoots' - where I can experience these alternate realities and timelines in my own ways through my own perceptions.

Now one thing to understand is - reality is consensus as long as it works for you. When it doesnt - choose you own reality - and rules - appropriately.

Be nice. That's a good motto to have as you do so.

And keep in mind. Science is there to help you imagine more. Not to confine you and your thinking as some would prefer.

The word universe is used to mean "All existing matter and space considered as a whole"; it is a term derived from Old French univers (12th century), which was derived from Latin universum meaning "all things, everybody, all people, the whole world." It is the noun use of adjective universus "all together, all in one, whole, entire, relating to all." If you translate it from the sources, literally means "turned into one," from unus "one" (see one) + versus, past participle of vertere "to turn" (see versus).

So if you want to say that the universe is everything which exists taken as a collective whole, then even *if* other universes exist as has been variously speculated in various multiverse conceptions, (you can see a list from wikipedia in the sources below) -- then all of these ideas are just redefining the word universe into new overloaded contexts, which ignore that original meaning.

If you are going to be precise, the word universe remains what it was in ages past -- everything considered as one thing, the ultimate superset. Trying to redefine it is splitting up everything, into subdivisions and since there does not appear to be any actual evidence to support that view, it's really just a kind of fantasy or imagination, a valid enough form of entertainment, but it's not a part of science at this point.

We have no evidence to prove or disprove this, all we can say is that what we have observed brings us to the conclusion that there is only one universe, but concerning more than one universe will stay a theory until proven or disproven

This is the only universe that we have evidence for its existence. Doesn't mean that it is the only one out there.

Can't get there from here. Other universes are pure speculation.

(1) Multiple Universe are simply a MATHEMATICAL 'model", They exist only in textbooks and smart(?) [people's heads

(2) if the DID exists we could never know it, and therefore they could have no effect on us.

If it is not connected to this universe it does not exist.

we do not know 1 or a 100