The quick answer is that, according to observations made by astronomers, there is no edge to the universe. The universe is constantly expanding meaning that space spreads out infinitely in every direction. Galaxies and nebulas and stars fill out all of the space throughout this infinite universe. There are two observations that make this believable.
Firstly, observations have shown that the parts of the universe we can actually see are both flat and uniform. This just means that, even though the universe is peppered with planets and galaxies, these are not close enough together to cause a non-uniform universe.
The universe is considered uniform on a cosmic scale. The flatness of the universe simply means that the spacetime that occupies the universe is not curved or warped in any way. Both of these things mean that the universe has no center, with by a center of mass or even by a center of expansion or curvature.
This flatness of the universe is a direct result of the uniformity of it. The universe does not bend around to curve back on itself and re-join another end.
Yes, it is true that objects like moons and stars can bend and warp spacetime thanks to their large mass, but in the wider cosmic scale of the universe, these are simply not large enough anomalies to alter the uniformity of the universe and its being flat.
The reason for this is that if you were to average out all of the moons, planets and galaxies etc. in terms of their distribution of mass, you would find a constant measurement.
Secondly, although humans tend to think that we are the center of the universe, we are not special in our little corner of the universe. If we comply with the idea that the rest of the universe is flat and uniform, it follows that your corner must be flat and uniform since it is no different.
This means that the only possible way for the universe to be truly flat and uniform is for it to have no edge. An edge would mean that it was not uniform. Although this is extremely hard to wrap our heads around, it is the only logical and sensical answer: the universe is both infinite and flat.
This theory suggests that if you were to float around space and pick a straight line to follow, you could go on forever without finding an edge.
The next question to pop up might be how the universe is infinite when it started out as a finite object in the Big Bang. Again, this assumption is wrong. The universe never started out as finite, our brains just cannot comprehend there being nothing and then something, so we imagine it as an object that exploded outwards.
This is not how the Big Bang worked. The Big Bang did not happen at one specific point in the universe, it happened everywhere and nowhere at once. That is the reason that astronomer can measure something called the flash of the Big Bang even after it happened.
We can see a cosmic microwave background radiation that exists everywhere in the universe, in every corner we can observe. This shows that the Big Bang was not an object expanding into what we know to be the universe today, it was actually the universe itself expanding into a larger version of itself.
Before the Big Bang, theories suggest that the universe already an infinitely large object, but that it expanded into an ever-larger infinite object thanks to the Big Bang. That is why we can see these remnants of background radiation everywhere in the universe. It is a perfectly valid concept in science for something that is infinite in size to actually increase in size.
As humans, we can only see a small part of the existing universe. This is what we like to call the observable universe. The reason we can only see a certain amount of the universe is mainly down to distance, but also light.
Light takes a certain amount of time to travel through distances. Some parts of the universe are so far away from us in the Milky Way that the light still has not yet reached us in order for us to discern them.
This light would have started out at the beginning of the universe just like our galaxy, but since it is so far away, the light simply has not reached us yet. This is profound since light is the fastest thing we know of, so nothing has been able to detect a signal in any way.
These places are still outside of our observable universe, but that does not mean that they do not exist. To other parts of the universe that we cannot see, it is important to remember that, should there be any life in those areas, they would not be able to see us either. We are outside their sphere of observation too.
In this sense, there is a sort of edge to the universe. Not the universe as a whole, but there will always be an edge to our observable universe. Nothing made of a physical barrier or a lip in spacetime, but simple the point at which the distance is too great for any light to reach us from far away galaxies.
In that case then, the observable universe may have an edge, but the real universe beyond does not have an edge since it is infinite and uniform.
But our observable universe is also increasing in size as time goes on, since the light from these unsees galaxies is eventually reaching us. This means that even the edge of out observable universe is obsolete, since it is changing all of the time. The annoying hitch in this solution is that the universe itself as a whole is constantly expanding.
This means that there could be a galaxy whose light we can almost see but thanks to the universe expanding, the galaxy is getting further and further away, meaning its light might never reach us. Some of the galaxies are so far away from earth that the expansion of the universe could mean they are moving away from earth faster than the speed of light.
Special relativity means that two objects local to one another cannot travel faster than the speed of light in relation to each other. But this does not work in the same way when universe expansion is involved. Imagine our galaxy and a faraway galaxy that is just outside of our observable universe.
Because the expansion of the universe is at play, these objects can travel faster than the speed of light away from each other. This means their light will never reach us, and thus, they will never become part of our observable universe. Therefore, the edge of our observable universe cannot actually keep up with the speed of the real universe’s expansion.
Even though we use the word ‘edge’ to describe the observable universe, it still stands that the universe itself has no edge since it is infinite and uniform, as well as eternally expanding.