Is it possible that our universe is a subatomic particle of a gigantic universe?

Yes, I’ve been tooting that horn for over four decades and I call it the nested universe model: our universe is a quantum in the next universe up in scale as every quantum in our universe is a universe much like ours, and so on forever and ever in both directions of scale. This description paints a pretty picture of infinity, moots questions about size, location, and causality (no “outside” for existence).

This model requires the universe to be closed, not open, so the inferred galactic recession commonly called “expansion” would be taking place in a geometry of spacetime which is totally curved. There is such a geometry, it’s the core geometric concept of the ToGR: Riemannian geometry. Years ago in a (naive) attempt to address a quandary presented to us from QM I made some drawings of quanta and it turned out to be Riemannian geometry. In order to be more descriptive those drawings had to be animated so I made this short video animation showing kinetic Riemannian geometry, you may find it interesting:

Depending upon how one models the universe and the electron, the equations are quite similar when the factors are adjusted for size. This was one of the correlations that promoted Dirac’s Large Number Hypothesis circa 1937. The ratio of the electron size to the Hubble size is approximately equal to the ratio of the strength of gravity field of an electron to the strength of the electric field of the electron

Here is what Carl Sagan had to say:

“There is an idea--strange, haunting, evocative-one of the most exquisite conjectures in science or religion..an infinite hierarchy of universes, so an elementary particle, such as an electron, would, if penetrated, reveal itself to be an entire closed universe. Within it, organized into the local equivalent of galaxies and smaller structures, are an immense number of other, much tinier elementary particles, which are themselves universes at the next level, and so on forever–an infinite downward regression, universes within universes, endlessly. And upward as well. Our familiar universe of galaxies and stars, planets, and people, would be a single elementary particle in the next universe up, the first step of another infinite regress.”

Before the beginning of time ‘The Big Bang” theory says the entire ‘Observable Universe’ was packed in a space of 10^20th i.e. the PLANK LENGTH which means our Universe itself was a small spec.

This maybe changed with further exploration but lets just assume it is totally real if this Universe was about the size of a spec, it is also possible there are other parallel Universes which may even be larger than ours.

The Universe we see now is just a part of which the light has traveled from far distances (light-years) and that is why called The Visible Universe. One cannot give the perfect answer for these questions. Yet, there is a possibility of our entire Universe is just a subatomic particle of an even more Gigantic-Universe.

Hope my answer helped with the question or just made it more tangled.

I’m not a particular apologist for Bang, but it does have the interesting property that a circumference of the actual boundary of (observable) Universe would be infinitesimal.

This has the implication that your latest exhalation could contain billions of ‘other universes’, other self-contained cosmic systems with their own isolated energy. These suystems are naturally separated from yours - they cannot be ‘parallel’ in the sense that we cannot communicate with their inhabitants, nor they with us. There would be an enormous energy barrier.

And, yes, in turn our own home cosmos could be an infinitesimal speck in someone (or something) else’s gaseous emission.

Maybe, but How do we know for sure?

Humans have still not even been able to get out of this Solar System yet.

The only things which the “Average Joes” like us can test, are the Newtons theories and those before that (eg. Galileo, Archimedes, etc.) and maybe a few others which came afterwards.

Others, (Darwin’s Evolution, Einsteins Relativity, Quantum Mechanics Theories, Super strings, etc.) were brought in to patch up the experimental anomalies (outliers) which occurred while testing previous laws and theories.

So, if you ask your question, a Hundred Professors may give you a Hundred different answers.