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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 evidence that the Universe began with the Big Bang is very compelling. 13.8 billion years ago, the entire Universe was compressed into a microscopic singularity that grew exponentially into the vast cosmos we see today.

But what does the future hold? How will the Universe end?

Astronomers have been pondering the ultimate fate of the Universe for thousands of years.

In the last century, cosmologists considered three outcomes for the end of everything, and it all depended on the critical density of the Universe.

If this critical density was high, then there was enough mutual gravity to slow and eventually halt the expansion.

Billions of years in the future, it would then collapse in on itself again, perhaps creating another Big Bang. This is known as a closed Universe, and the final result is the Big Crunch.

If the critical density was low, then there wouldn't be enough gravity to hold things together.

Expansion would continue on forever and ever.

Stars would die, galaxies would be spread apart, and everything would cool down to the background temperature of the Universe.

This is an open Universe, and the end is known as the Big Freeze.

And if the critical density was just right, the Universe's expansion goes on forever, but it's always slowing down, reaching a dead stop in an infinite amount of time.

This creates a Flat Universe... also a Big Freeze.

Fortunately, astronomers were able to measure the critical density of the Universe, using NASA's WMAP spacecraft, and they discovered that the actual density of the Universe predicts a flat Universe.

So that's it, right?

Of the three choices, the answer is #3.

Unfortunately, nature had other plans, and came up with a reality that nobody expected.

In 1998, a team of astronomers were observing distant supernovae to get a sense of how fast the Universe is slowing down and they made an amazing discovery.

Instead of decelerating, as predicted by the critical density of the Universe, the expansion of the Universe is actually speeding up.

Some mysterious force is pushing galaxies faster and faster away from each other, accelerating the expansion of the Universe.

We now call this force "dark energy", and for the time being, astronomers have no idea what it is.

All we know is that it's pushing the Universe apart.

Distant galaxies are being accelerated away from us, and in trillions of years from now, they will cross the beyond the cosmic horizon and disappear from view.

The evidence that we live in a vast Universe will disappear with them.

But there's a further unsettling possibility about dark energy. Maybe the expansion pressure will increase, eventually overwhelming gravity on a local level. Galaxies will get torn apart, and then Solar Systems, and eventually atoms themselves will be shredded by the increasing dark energy - this idea is known as the Big Rip.

So how will the Universe end?

The force of dark energy will continue to accelerate the expansion of the Universe until distant galaxies disappear.

Galaxies will use up all the gas and dust for stars and go dark, perhaps becoming black holes. Those black holes will decay and maybe matter itself will decay into pure energy.

The entire Universe will become a cold, quiet place, where single photons are stretched across light years of space.

Don't worry, though, that won't be for quadrillions of years from now.