We don’t know. But whatever it is, it’s big.
I was chatting once with Michael Anissimov and somehow we got onto the subject of parallel universes, and whether we could ever know anything about them. Michael’s answer was no, we can’t. By definition, he explained, a separate universe is an entity we can’t get any information about. It is something that lies completely outside our knowledge.
As described in the linked article above, dark flow is a mysterious force that is mysteriously accelerating the movement of some galaxies in a particular direction. One explanation for dark flow is that there is some massive megastructure exerting gravitational pull on these galaxies, drawing them to the very edge of the universe.
In fact, one theory is that the megastructure is, itself, a separate universe.
Aha, but if we know that very large thing is out there because it’s exerting force in this universe, doesn’t that mean that it can’t be a separate universe?
I know the confusion results from using two different definitions of “universe.” That megastructure might be a separate massive collection of galaxies that had its own Big Bang…whenever. But it’s all part of our same universe, which is therefore much, much bigger than we’ve been thinking.
Of course, it doesn’t have to be galaxies. It could be anything. All we know for sure is that, if it is in fact there, it’s really, really big.
And by big I mean massive.
UPDATE: In response to Zapboomer’s comment, there is a non-zero — but I’m going to say extremely remote — possibility that the megastructure beyond the edge of the visible universe exerting the force on those galaxies is, in fact, a giant Sasquatch. It would have to be made up of different stuff than the ones we have here on earth, or subject to different laws. But it can’t be ruled out altogether. If there, it might look something like this:
However, I’m going to say that another universe is more likely.
