In physics, the word "universe" has a few different definitions. One of these suggests that in a very real way, we all inhabit our own personal universe.
One practical definition, the "known" universe, is everything we've been able to see. The known universe gets bigger in fits and starts, every time we build a better telescope. It has irregular borders – if there's a nebula or a galactic center in the way, we might not be able to see as far in some directions as others.
A perfect telescope would have a maximum limit, called the "observable" universe. In order to see something, the light has to have had time to travel here. The observable universe defines a sphere around Earth 14 billion light years across. That's the furthest away we can possibly see, because that's the oldest light arriving.
There's still regular stuff on the other side of the boundary: more stars, nebulae, galaxies. It's fair to call this border "imaginary". But physics points out a deeper significance: if something can't be observed, then it can have no possible effect on you. It might as well not be there at all.
Imagine a star 14 billion years away that blinks into existence, emits one single photon, and then shuts down. Depending where we are on Earth, you might be close enough receive that photon and I might be too far. It's possible I can be affected by events that have no reality to you, and vice versa.
14 billion light years is such a great distance, we "round off" and act as if there is only one observable universe, whose center is the Earth. But every observer has their own boundary.
These individual universes overlap almost completely. But "almost completely" does not mean "identical".
Science describes the overlapping universe, the world we share outside of our heads. Inside, we describe our own universe in the language of art: myth, metaphor, spirituality.
I believe the next great frontier in Science lies in studying the internal reality. Neuroscience makes the first attempt at applying physics to the brain. Imagine instead measuring the interior world by the position of the stars, or collapsing the wavefunction of a lifetime.
A Science of the humane.