Its no doubt the universe is one of the greatest marvel of all time. Its a concept that has intrigued and confused so many astrologers, physicist, and religious fanatics to mention a few.

But One fact that is really mind boggling is the fact that the universe is constantly expanding;

for every million parsecs of distance from an observer, the universe expands at the rate of 67 kilometers per second.

But it begs the question;

If the universe is constantly expanding, then what is it expanding into, what is outside the universe that allows it to expand, why is the universe expanding, where is the edge of the universe, how is this expansion really like? Can you wrap your head round these questions for a moment and in that moment remember that the universe has probably expanded with over 2804 kilometers.

So lets take a look at:

SOME OF THE THINGS YOU NEED TO KNOW ABOUT THE EXPANSION OF THE UNIVERSE

The Metric Expansion Of Space

The metric expansion of space is the increase in the distance between two observable points in the universe with time. You would note i said observable, this is cos there is a limit to what we can observe in the universe due to the slowness of the speed of light in relation to the distance at which the universe expands.

Note that nothing can move faster than the speed of light, so pardon the statement "slowness of the speed of light" however it is possible for things to be so far away from each other so that as space expands, distances would seem like it is faster than the speed of light.

Wikipedia

So what this means is that, the scale of space is constantly changing, implying that the universe did not expand into anything, instead space itself changed carrying the universe with it as it grew.

Think of it this way;

Imagine 2 flies standing on an elastic band at different points, now imagine that the elastic band is stretched at both end at the same time. You would notice that the distance between the two flies would start to increase even though they are not moving. Now lets imagine that Fly A is trying to send a message to fly B, as the elastic band stretches, the time it would take the message to go from fly A to Fly B slows down in relation to the stretch.

This my friend is a fisherman explanation of the metric expansion of space.

The Two Observational Evidence Principle

It should be noted that metric expansion of space is just one of the numerous feature of the universe and their are 2 major principles under pinning the several features of the universe.

1. The Cosmological Principle: This principle states that the universe looks the same way in all directions and has roughly the same smooth mixture of material

2. The Copernican Principle: This principle states that no place in the the universe is preferred and that the universe has no starting point.

THE GALAXIES ARE SHIFTING AWAY FROM US

In 1925, American astronomer Edwin Hubble showed that there were many separate island of stars and they are mostly millions of light years away from us. He also discovered with the aid of very powerful telescopes that light coming from these stars and galaxies shifted towards the red end of the spectrum due to something known as the Doppler Effect.

With this discovery, Hubble was able to prove that the galaxies were moving away from us. Not just that, by studying a special class of stars called CEPHEIDS, he concluded that the galaxies were moving away from each other at great speed.

An impression of this expansion

From the image above, the four spiral dots represents clusters of galaxies and you can see from the initial base they are together, but as time passes and moving upward you would see that they are beginning to go further apart and expanding.

This expansion is a “broad-brush effect” such that galaxies themselves are not expanding but clusters of galaxies in which matters in the universe become divided and get widely seperated and more thinly spread throughout space.

Therefore the universe is not expanding outwards into pre-existing space, its space itself that is expanding.

Despite this expansion, the universe would still look more or less the same from every galaxy. In essence, even though we see all the galaxies receding from us, that does not necessarily mean we are at the center of the universe

In such an expansion, the universe continues to look more or less the same from every galaxy, so the fact that we see all the galaxies receding from us does not necessarily mean that we are at the very center of the universe; if there are observers in other galaxies they would also see all the other galaxies going farthest away according to this principle.

SO WHAT IS THE UNIVERSE EXPANDING INTO

The truth of the matter is the universe is not expanding into anything, it is just expanding based on space, space is stretching. And since the universe is everything if there was something to expand into, then that thing is also part of the universe.

Source

The universe is not like a house that you can go out of or a box. The universe is all that exists so you cannot be outside of it. The world as we know it is three dimensional, however the universe is 4 dimensional so trying to understand the concept of the universe expanding into something is like trying to a needle in a basket of needles.

The universe isn't "contained within" anything, as humans would visualize it. The universe is all that exists so you cannot be "outside" of it, like we think of being outside of a house.We evolved to deal with things in our immediate three-dimensional world; we literally lack the biological capacity to comprehend the true nature of the universe."Outside the universe" is a concept we simply can't grasp.

The simplest way to understand this is to imagine the universe is a balloon and that balloon is all that there is. If you paint two points on that balloon and inflate it, the distance between the two point stretches as the balloon expands. So in a 3d sense, the universe is expanding when in truth its just the distance between this two points that's expanding.

WHAT IS OUTSIDE THE UNIVERSE?

Again the simple answer in nothing. The main issue with this question is cos we think of the universe the same way we think of things on earth. The universe doesn't work that way. Outside the universe quite literally doesn't exist. Cos the universe is outside and in itself.

All of everything that exist is the universe, if you think there is something outside the universe, then that thing is in itself part of the universe.

REFERENCES:

1. The Metric Expansion of Space.
2. The Expanding Universe and Hubbles's Law
3. Accelerating Expansion of the Universe