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I get this question... a lot.

"Since astronomers know that the Universe is expanding, what's it expanding into? What's outside of the Universe?"

The short answer is that this is a nonsense question, the Universe isn't expanding into anything, it's just expanding.

The definition of the Universe is that it contains everything.

If something was outside the Universe, it would also be part of the Universe too.

Outside of that? Still Universe. Out side of THAT? Also more Universe. It's Universe all the way down.

But I know you're going to find that answer unsatisfying, so now I'm going to break your brain.

Either the Universe is infinite, going on forever, or its finite, with a limited volume.

In either case, the Universe has no edge.

When we imagine the Universe expanding after the Big Bang, we imagine an explosion, with a spray of matter coming from a single point. But this analogy isn't accurate.

A better analogy is the surface of an expanding balloon. Not the 3 dimensional balloon, just its 2 dimensional surface.

If you were an ant crawling around the surface of a huge balloon, and the balloon was your whole universe, you would see the balloon as essentially flat under your feet.

Imagine the balloon is inflating. In every direction you look, other ants are moving away from you. The further they are, the faster away they're moving.

Even though it feels like a flat surface, walk in any direction long enough and you'd return to your starting point.

You might imagine a growing circle and wonder what it's expanding into. But that's a nonsense question. There's no direction you could crawl that would get you outside the surface. Your 2-dimensional ant brain can't comprehend an expanding 3-dimensional object.

There may be a centre to the balloon, but there's no center to the surface. Just a shape that extends in all directions and wraps in upon itself.

And yet, your journey to make one lap around the balloon takes longer and longer as the balloon gets more inflated.

To better understand how this relates to our Universe, we need to scale things up by one dimension, from a 2-d surface embedded in a 3-d world, to a 3-d volume embedded within a 4-d universe.

Astronomers think that if you travel in any direction far enough, you'll return to your starting position. If you could stare far enough into space, you would be looking at the back of your own head.

And so, as the Universe expands, it would take you longer and longer to lap the Universe and return to your starting position.

But there's no direction you could travel in that would take you outside or "off" of the Universe.

Even if you could move faster than the speed of light, you'd just return to your starting position more quickly.

We see other galaxies moving away from us in all directions just as our ant would see other ants moving away on the surface of the balloon.

A great analogy comes from my Astronomy Cast co-host, Dr. Pamela Gay.

Instead of an explosion, imagine the expanding Universe is like a loaf of raisin bread rising in the oven.

From the perspective of any raisin, all the other raisins are moving away in all directions.

But unlike a loaf of raisin bread, you could travel in any one direction within the bread and eventually return to your starting raisin.

Remember that our entire comprehension is based on 3-dimensions. If we were 4-dimensional creatures, this would make much more sense.

For a much deeper explanation, I highly recommend you watch my good friend, Zogg the Alien explain how the Universe has no edge.

After watching his videos, you should totally understand the possible topologies of our Universe.

I hope this helps you understand why there's no answer to "what is the Universe expanding into?"

With no edge, it's not expanding into anything, it's just expanding.

The Universe is expanding. Distant galaxies are moving away from us in all directions.

It's natural to wonder, is everything expanding?

Is the Milky Way expanding?

What about the Solar System, or even objects here on Earth. Are atoms expanding?

Nope.

The only thing expanding is space itself.

Imagine the Universe as loaf of raisin bread rising in the oven.

As the bread bakes, it's stretching in all directions - that's space.

But the raisins aren't growing, they're just getting carried away from each other as there's more bread expanding between them.

Space is expanding from the Big Bang and the acceleration of dark energy.

But the objects embedded in space, like planets, stars, and galaxies stay exactly the same size.

As space expands, it carries galaxies away from each other.

From our perspective, we see galaxies moving away in every direction.

The further galaxies are, the faster they're moving.

There are a few exceptions.

The Andromeda Galaxy is actually moving towards the Milky Way, and will collide with us in about 4 billion years.

In this case, the pull of gravity between the Milky Way and Andromeda is so strong that it overcomes the expansion of the Universe on a local level.

Within the Milky Way, gravity holds the stars together, and same with the Solar System.

The nuclear force holding atoms together is stronger than this expansion at a local scale.

Is this the way it will always be?

Maybe. Maybe not.

A few decades ago, astronomers thought that the Universe was expanding because of momentum left over from the Big Bang.

But with the discovery of dark energy in 1998, astronomers realized there was a new possibility for the future of the Universe.

Perhaps this accelerating dark energy might be increasing over time.

In billions years from now, the expansive force might overcome the gravity that holds galaxies together.

Eventually it would become so strong that star systems, planets and eventually matter itself could get torn apart.

This is a future for the Universe known as the Big Rip.

And if it's true, then the space between stars, planets and even atoms will expand in the far future.

Is this going to happen?

Astronomers don't know.

Their best observations so far can't rule it out, or confirm it.

And so, future observations and space missions will try to calculate the rate of dark energy's expansion.

So no, matter on a local level isn't expanding.

The spaces between planets and stars isn't growing.

Only the distances between galaxies which aren't gravitationally bound to each other is increasing.

Because space itself is expanding.