"Nature never deceives us; it is always we who deceive ourselves."
Jean Jaques Rousseau
The way of the universe that I have presented focuses primarily on what is happening now and what appears to have been happening for about 8 billion years. This does not preclude other possible scenarios for what happened earlier or what will happen later, but it does provide a potential explanation for what has happened for an eternity in the past and for the future of the universe.
To be clear, the stability of the density of matter, given an expanding and flat universe, indicates that mass is turning into galaxies at a rate the exactly matches the rate of expansion of the universe. That says nothing about whether there was a big bang of some kind, or how old the universe is, but it makes a bang unnecessary. Occam's razor would remove the baggage of a Big Bang since it contributes nothing to the picture of a universe that would otherwise be self-sustaining. Nonetheless, should sufficient evidence become available to show that the universe does indeed have a beginning (rather than inferred from faulty premises), then the universe had a beginning. If the James-Webb telescope confirms that there are no galaxies beyond (before) 13.2 billion light-years, then the universe has indeed changed dramatically since then.
It would be premature to speculate about what a big bang might have been if mass is being added to the universe, but there is no reason to say it couldn't have been similar to what has been accepted in the literature. We will never know the true extent of the universe however because we can't know the extent of matter prior to the "last galaxy" (or first galaxy, depending upon whether you are referring to distance or time). It seems extremely unlikely, however, that the universe consists only of what we are able to see from Earth. Assuming a flat universe and the validity of the Cosmological Principle, an infinite universe is most likely, although that is hard to reconcile with any beginning.
Is it possible that the new galaxies that must have been added to maintain stable density of matter formed from preexisting materials that have existed since a beginning? That, in fact, is current theory, however it would be a remarkable coincidence that the rate of formation of galaxies added exactly matched the rate of expansion of intergalactic space. It suggests that, instead of a rapid period of galactic formation, there has been a rather gradual rate of formation at a plateau that mirrors galactic expansion. This would imply that the plateau would eventually end.
The fate of the universe faces similar conundrums. If the matter in the universe is fixed, then the available matter would eventually be depleted, and the universe would die a heat death. A stable rate of expansion (or accelerating but limited by the amount of mass being formed) matched by the formation of mass does not have to have an end at all - at least not for the whole universe. Everything in it may eventually disappear but be replaced by new matter in the same way that an army continues to exist for longer than people live. But although the rate of formation of galaxies has maintained a stable density of matter thus far, there is no guarantee that this will continue indefinitely - although there is also no reason to think it would not.
It is reasonable to wonder if what we are seeing now (and for the period of time available for our inspection in the past) is the same in areas where we are unable to see because of the rate of recession. What lies outside of the Hubble radius? Given the Cosmological Principle and Copernican Principle, the most reasonable answer is that things are pretty much the same on the other side of that impenetrable barrier. Impenetrable to us, that is, but not to the galaxies that we can see in the distance.
Without any evidence of other dimensions, there is no need to include them in the discussion, but that may depend on the definition of dimension. Something existing outside of our perception, and even outside of our ability to perceive, may be a type of dimension. Given that every galaxy affects every adjacent galaxy, there is probably an unending chain that is at once connected and yet parts of the chain are forever hidden to other parts. Maybe it's turtles all the way down.
In the end, even though there is nothing that can be said about the whole universe from experience or experiment, nature tends to operate by patterns that repeat. Still, infinity is a long way, and eternity is a long time in either direction.
