This question actually doesn't make sense, it's kind of a paradox in the same way the question of what happened before the Big bang is also strange in the sense that the universe and reality didn't exist in a form with causality in effect.
So asking a "before" question in reference to "before" time even started is paradoxical in and of itself. Since "before" wasn't even a concept in existence.
Which is why scientists don't really worry about anything "before" the Big bang.
The Universe is expanding, rapidly from the big bang still. At some point, it will slow down, and then stop. Then begins a catastrophic cycle of collapse with massive black holes coalescing into one universe eating black hole that compresses every bit of matter into a single point of almost infinite density. At this point the black hole destabilizes, and all of the stored energy is released in one colossal explosion. A Big Bang of sorts.
All of the current scientific evidence disagrees with this. 1) There is a velocity such that you can go faster than gravity will be able to slow you down: escape velocity. So, it's possible even without any new, weird physics. 2) The hubble constant shows that the universe isn't slowing down, but the opposite: it's accelerating. Physics doesn't know why (see Dark Energy). It's physically measurable that things farther away are accelerating even faster scaling with distance.
There's no proof the universe will end in a Big Crunch. Apparently there's some measure of the universe where if it's less than 1, we'll get a Big Crunch, and if it's greater than 1, we'll get a Big Rip where everything just falls apart. I may have those backwards, but the important thing is when it's exactly 1, it implies a universe that continues forever, getting colder and colder. And as best as science can determine for our universe, the value is precisely that.
But here's another, well, dimension to that: There's a popular but unprovable conjecture that our universe is the inside of a black hole that exists in a higher-level universe. In our universe, black holes boil away due to Hawking radiation, a process that can take trillions of years for very large black holes.
Once the black hole we're inside of stops consuming matter in the level above, that spells a very slow but alternative end to our universe. One day it will simply cease to exist.
"This the way the world ends: Not with a bang, but a whimper." -- T.S.Eliot.
I've become a fan of the "We're already in a black hole" theory. The Schwarzschild radius for the mass of the known universe is larger than the radius thereof.
I’m a big bang denier. I have zero evidence. I believe everything has always been, will always be, and goes on forever in every direction. I think anything we do to try to explain is just to protect our brains from being incapable of fathoming that everything is infinite.