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Oh boy, where to start on this. I started writing a point-by-point response, but you'd do better to read some books on the topics. You've taken a few facts out of context, gotten others wrong, and misunderstood pretty much everything.

By the way, the observable universe is not 46.5bn light years across, it is actually about 90bn light years across. You were thinking of the radius, the distance from us to any point on the horizon.

This is possible for a few reasons. First, the Big Bang did not come from a singularity. It happened everywhere, all at once. The early universe was already a significant portion of its current volume.

Second, space itself expands. The rate of expansion of space is accelerating. This is where Dark Energy (not related to Dark Matter) ties in. The expansion of space accounts for why the radius of the universe is greater than 13.8bn light years, and why the horizon is effectively receding faster than the speed of light. The expansion of space is contributing to the red-shift that we measure to determine how fast objects are moving.

Dark Matter has little to do with hypotheses of FTL travel. Dark Matter is as-yet unobserved mass that appears to form primarily in halos around many, but not all, galaxies including our own. While we have not directly observed it, we still have significant observational evidence of its existence. We might never directly observe it as there may not be enough of it around the Earth for us to get usable data about it. It may even be repulsed by normal matter, in which case we really will never directly observe it.

There I go, writing a response when you'd really do better actually reading credible books on the topics. Maybe start with A Brief History of Everything by Bill Bryson. A lot of his book is not quite correct, but it is close enough and you can go from there to figure which parts are accurate and which are not. Just taking a few popular works from my own bookshelf I'd recommend In Search of Schrodinger's Cat by John Gribbon, The Particle at the End of the Universe by Sean Carroll, The Character of Physical Law by Richard Feynman, and The First Three Minutes by Steven Weinberg. These are all decent popular works as I recall. They won't replace a an actual undergraduate education, but they will at least give you a general introduction to cosmology, particle physics, and more importantly the processes and history of the science of physics. You could also benefit if you read some works on rational skepticism to help separate out the bullshit.

Also read some of the biographies and popular works written by the legends in physics, but keep in mind that even the greats had their flaws and logical failures. Stay away from anything that tries to tie quantum physics to the supernatural, or string theory in general because that is rich territory for nonsense. Even some of the mainstream theoreticians in string theory are nuts.

The speed of light in a vacuum is a fundamental physical constant, the currently accepted value of which is exactly 299,792,458 metr. es per second, or about 186,282 miles per second.

Incredibly untrue. Just look at the age of the universe and how many light years apart is is. Ridiculous. So many base so many things on incorrect measurements and false ideas. It is unreal. Pun intended.