[kasparovitch (OP)]

To the best of my knowledge, immediately outside a univerese, like ours, there is nothing.

That is outside it there's no matter, no time, nor even dimensions.

Thus 2 universes cannot be referenced one to the other as there is no way to compute distances outside each of the universes. It only exists in each of them, not outside them.

However universes are know to be in expansion, at leat ours and perhaps all others, if there are any.

So my question is, can a universe in expansion ever overlap another?

Of course, this question admits first of all the multiverse hypothesis.

Cosmologists studying a map of the universe from data gathered by the Planck spacecraft have concluded that it shows anomalies that can only have been caused by the gravitational pull of other universes (In: Article by Rosie Taylor for the Daily Mail, 19 May 2013).

This is paradoxical in my opinion...

[evan_au]

Nobody really knows if multiple universes exist (and there are multiple theories of how they could exist).

One extremely simplistic model of multiple universes is two black holes. We know of no way to get from inside one black hole into the other.

But two black holes can merge, and the previously-separate universes now become united.

[IAMREALITY]

Unfortunately this is quite an impossible question to answer, and literally any and all answers you get would have an equal weight of credibility I would think.

But many do in fact believe they could overlap.  I'm not sure myself how I feel.  I guess, the way I see it, is that we're talking about two distinct spacetime systems.  Each of those systems may very well have their own rules of physics.  But since everything within a universe is contained, embedded, within its own spacetime, I would think the relevant question as to the possibility of multiple universes overlapping would simply come down to how spacetime itself interacts with itself.  We know how matter interacts with matter, energy with energy, etc.  But how does spacetime interact with a separate and distinct spacetime?  No idea what sort of theories if any might have been done on such a topic, but I would think that's the core question that would need to be answered.