When people say “the edge of the universe” they actually mean “the edge of the visible universe”. Why? It’s a combination of the (constant) speed of light and the rate that objects (galaxies for this example) fly away from each other in the universe.

Say a galaxy beside us is moving away at 2 kilometres (we’ll use km, 1000m, instead of miles as miles doesn’t really fit in with science very well) per hour, then another galaxy further on is moving at 4 km per hour from our viewpoint in galaxy 1. Galaxy 2 sees both galaxy 1 and 3 moving away from it at 2 km per hour, and galaxy 3 has a similar view to galaxy 1, but for it it looks like we’re the ones travelling away from it at 4 km per hour.

And as these galaxies move away from each other, that 2 km/hr is no longer true, and speeds start to get faster and faster.

So objects in the universe are not just moving away from each other, they are also accelerating away from each other.

Now let’s take that logic and take us thousands of km/ph, then millions of km/ph, then billions of km/ph, and beyond…

Then we get to the speed of light. The speed of light per second is easy to find but gets even more massive when talking about light in km/hr.

1.08 x 10 to the power of 12 km/hr or 1,080,000,000,000 km/hr

That’s a trillion km/hr. And we’re now at the edge of the visible universe - 46.5 billion light years away.

When objects are flying away from us an inch / cm / etc faster than that, we will never see them again. And of course, that object is continually accelerating, so of course it is going faster than that already.

And why don’t we see them? If an object is moving away from us faster than the speed of light, then the light emitted by that object will continue to be emitted, but it will never ever reach us … it didn’t and it can’t go fast enough.

What we see as things get closer to the edge of the universe is that they get massively red shifted, then when they get to the edge, it’s a bit like a black hole - things will appear to get fainter, then as they approach and cross horizon, they will infinitely slow down and appear to stop, then fade away. Or I assume they can’t fade completely, so all objects that ever passed the horizon will be visible on it, just incredibly faint, and effectively gone.

That galaxy that has just sadly disappeared from our view is … ok (or at least it wasn’t affected by a visual effect only experienced by us). That galaxy will have it’s own visible universe, and has just seen our Milky Way slow down, stop, and get continually fainter. And similarly, every object in the very likely infinite universe (very likely, as we can’t observe it for ourselves, snd never will) will experience it’s own individually experienced visible edge of the universe.

Over time, we will see more and more galaxies travel over that horizon, and one day in the far flung future, we’ll not see any other galaxies or objects outside of the milky way (or Milky Way and a few other galaxies linked together by a gravitational force stronger than the expansion rate - these galaxies including the milky way are known as the Local Group). Beyond the stars of the Local Group, it will be dark, and it always will be.

Will that force pulling apart galaxies impact the galaxies themselves - will galaxies be destroyed as stars find that expansion force to be higher than the gravitational force acting on them?

The current theory here is that the expansion rate isn’t big enough for that to happen, but scientific debate changes - or is enhanced - in this field all the time. It’s really interesting stuff!

"They" cant tell us because nobody knows. Chances are, it'll always be impossible to know. It doesnt matter that much though, whichever answer it is, its gonna be too weird to wrap our heads around. That's already the case with a lot of physics we do somewhat understand.