If our universe is expanding due to 'the big bang,' what could possibly stop its expansion?

Your question misinterprets the the expansion of the universe as expanding INTO space. The actual expansion is with the SPACE ITSELF.

Very nice question. The universe is not only expanding but it is accelerating in its expansion. @stanleybmanly Is correct that it is expanding into space itself. It’s a difficult concept to grok, but there will never be anything to “stop” its expansion. At least as far as we know now.

And the space is being created by the expansion of the universe, and we can measure that expansion as though we were a dot on the surface of an expanding balloon watching all the other dots recede from us. There is no center. In the case of our universe, the big question remains about the amount of mass in the universe which should determine whether the universe will slow in its expansion due to gravity to the point that it will then contract or instead expand indefinitely. Back when I was growing up, the debate was whether the universe was infinite, steady state or finite but unbounded like the surface of the globe. The “shape” would depend on the curvature of space, once again determined by the amount of mass. Then came the discovery that instead of slowing down in its expansion as Newton says it must, the process is speeding up, meaning there is impossibly more matter than we can account for presently, and everyone’s confused.

What could reverse the expansion caused by the big bang?
A giant black hole, creating the Big Suck.

It was thought that gravity might slow and eventually stop the expansion of the universe but the expansion of the universe appears to be inexplicably speeding up. No one knows why.
We do know that the centre of the universe isn’t the earth, or the sun it is everywhere, because every point in space can trace its origins to the big bang.

The universe is not expanding into space, space itself is expanding, and the universe with it.
It was thought that the combined gravity of all matter in the universe would slow down the expansion, stop it, and then reverse it, eventually compressing the universe back into a single point.
But observations tell us that the expansion is actually accelerating, and we do not know why. The cause of that excess energy responsible for the acceleration is called “Dark Energy”, so called, because no one knows what it is.
In addition, it was also thought that the combined gravity of a galaxy is what prevents it from disintegrating from its own spin.
But it turns out that galaxies appear to be a lot heavier than the combined mass of all the stars within them, because based on that combined stellar mass, galaxies are rotating too fast, and should be spinning themselves apart.
That unaccounted, invisible mass is called “Dark Matter”, again called so, because no one knows what it is.

There is still a lot that we do not know.

The simple answer: nothing. There is no finite or predictable limit to expansion of the universe.

As impressive as the theories of astrophysicists about the universe outside out solar system (let alone outside our galaxy, let alone the entire universe) are, the rate at which those ideas change and vary, suggests to me that those ideas will continue to change, indicating that, as @ragingloli wrote two answers up, we do not know.

And the level at which non-astrophysicists actually understand the fullness of even those theories, seems to me even lower.

First of all, assume that the Great Void had always existed, as well as all matter within the void.
I think some assume that there was absolutely nothing, and then, BANG, everything, including the void, came into being.
Since time has no beginning and no end, like the universe, there may have been countless Big Bangs, IMO.

“If our universe is expanding due to ‘the big bang,’ what could possibly stop its expansion?”

“When the big bang went bang, there had to be space already present for all that expansion to take place.”

Of course, the universe is expanding into space. That theoretical mass that exploded could not explode without a void (unoccupied space) to expand into.

I couldn’t spell astrophysics without a spell checker, so I am limited to asking the questions or trying to state what I can perceive on my own. But it seems that in order for the universe to reverse into a state of contraction, there would need to be a force greater than the mass of the universe itself in order to overcome its inertia and return everything again to that pea-sized or whatever original mass.

I think that science does its best to realign its position as new facts are learned. There must be some way that someone can explain these things so that even someone like me can understand it.

When I was just a little tyke in Sunday school, I asked, “Where did God come from?” The answer was, “He always existed.”

When I was an adult tyke, I asked the physicists, “Where did that stuff come from that went bang?” The answer was, “It always existed.”

Insufficient responses from both fields. There are 31,102 verses in the King James version of the bible. There are no mathematical formulas or scientific inquiry that can expand on that. The many fields of science, however, continue to investigate, prove or disprove theories, and continue to investigate what is and what may be. I will have to stick with the scientists, whether or not they are heretics, as they seem to be the ones expanding our knowledge of all things.

How many laws of physics does the big bang theory violate?

“When I was an adult tyke, I asked the physicists, “Where did that stuff come from that went bang?” The answer was, “It always existed.”” Sources please .

Astrophysicist on the Science Channel say they,“don’t know what was there before the Big Bang!” There wasn’t “things” for several seconds after it the big Bang only plasma. Hydrogen nuclei and Helium nuclei formed from the plasma but no atoms appeared for 380,000 years after the Big Bang