This discussion will certainly have logical flaws. It will be circular -- it will be messy. How do we define "existence"? How do we define what is "real" and what isn't? I will informally take up this topic.
First, I'll grant that the material universe that we observe is "real." When someone throws a rock at you and hits you in the head, that's real. Any definition of reality that denies this seems to me to be useless. But of course it's not as easy as we think.
I'll defer a discussion of our inner psychological experiences until later. On some level feelings, dreams and thoughts are real -- but more on that later.
This topic is circular, I think, because it aims to find a formulation to satisfy a pre-existing concept of what is real. we expect certain qualites to that which is real, and so search for a definition that fits those qualities. It's a tautology of sorts.
Why struggle to define what's real? I wouldn't except for the question of its origins. Yet I suspect some circular reasoning here also.
From the view of sciences, we can look at the Big Bang theory, which doesn't explain creation, but does describe the early universe back to after the Planck epoch (here's Wiki on the PE):
"In
physical cosmology, the Planck epoch (or Planck era), named after
Max Planck, is the earliest period of
time in the history of the
universe, from zero to approximately 10−43 seconds (
Planck time), during which
quantum effects of gravity were significant. One could also say that it is the earliest moment in time, as the Planck time is perhaps the shortest possible interval of time, and the Planck epoch lasted only this brief instant. At this point approximately 13.7 billion years ago the force of gravity is believed to have been as strong as the other
fundamental forces, which hints at the possibility that all the forces were unified. Inconceivably hot and dense, the state of the universe during the Planck epoch was unstable or transitory, tending to evolve, giving rise to the familiar manifestations of the fundamental forces through a process known as
symmetry breaking. Modern cosmology now suggests that the Planck epoch may have inaugurated a period of unification or
Grand unification epoch, and that symmetry breaking then quickly led to the era of
cosmic inflation, the
Inflationary epoch, during which the universe greatly expanded in scale over a very short period "
At T=0, the instant of creation, physics currently has nothing to say, much less at T<0. So it's a philosophical or logical problem, not a scientific one.
One could take the view that creation is not necessary, that reality begins with t=0 (my shorthand) -- and there's a certain sense to it. We exist in the spacetime continuum, and there is no "outside" beyond it.
Yet is that satisfying? Somehow we strive to ask "where did this STC (space time continuum) arise from?" That's the endless Colbert painting paradox. Whatever the answer is, it only begs the question "so where did THAT come from?" ad infinitum.
One answer would be an infinite series of nested universese, like taking the colbert portrait out to infinity. But that just seems evasive -- still the question would be "where did this infinite series of nested universes come from?" Even an infinite Russian doll has to exist IN something.
Or does it?
