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The universe is huge and getting bigger all the time, and we have we have dark energy - the most mysterious force in the universe - to blame/thank for it. Thought to make up more than 70% of the energy in the whole universe, Hank describes how dark energy was theorized to exist, and how scientists are trying to explain it.

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As you are probably aware, our universe is huge. But what you might not be aware of is that it's getting huger all the time, and for that, we have dark energy, the most mysterious force EVER, to blame-slash-thank.

So the universe is expanding. Not only that, but the farther away you go in any direction, the faster everything's flying apart from everything else.

Now, we've known since 1929 that the universe was expanding; that's when astronomer Edwin Hubble (you recognize that name) noticed that light from other galaxies was shifted toward the red end of the spectrum, indicating that they were flying away from us.

But it wasn't until 1998 that astronomers observing distant supernova discovered that the universe was actually expanding at an ever-increasing rate the farther away we look.

Now this is really counter-intuitive, given everything we know about the Big Bang and our old friend gravity. As you've no doubt heard, the Big Bang Theory suggests that the universe came from a single, unfathomably dense point, and the crazy amount of energy generated by the Big Bang caused everything - space, time, matter, everything - to come into being, and radiate out from that point.

And for about 3.5 billion years after the Big Bang, things went along about as you'd expect. The expansion of the universe gradually slowed as gravity tried to pull all the matter in the universe back together.

But then, something weird happened.

Another force began to overpower the pull of gravity between all the matter, and made the expansion of the universe accelerate.

SPOILER ALERT: that's what we call dark energy, thought to make up more than 70% of the energy in the whole universe.

But as bizarre as this is, as physics go, it isn't totally out of left field, because Albert Einstein actually predicted it in 1917, when he was first trying to apply his new theory of general relativity to the structure of space and time.

Einstein found that his equations didn't work for a static universe, so he threw in a hypothetical repulsive force that he called the cosmological constant, in order to fix his math problem. This constant was basically a certain density of energy that was theorized to permeate the universe.

The funny thing is, once Edwin Hubble discovered the universe wasn't static and was, in fact, expanding, Einstein thought that the cosmological constant was no longer necessary and he scrapped it.

But now, the concept is back on the table as a way of explaining dark energy. So what is now known as the cosmological constant theory assumes that dark energy is just a fundamental property of space. In keeping with Einstein's thinking, it proposes that empty space is not nothing, it has properties like anything else.

One property of empty space is that it can grow, more of it can be made. Another property is that it can possess its own energy, and if dark energy is a property of space, as more space comes into existence, more dark energy would appear.

Another theory that some are pushing is called quintessence, named after the fifth element of Greek philosophers. Quintessence suggests that dark energy is a new type of energy that is temporarily filling space, but it might eventually fade away just as it arose.

And there are other ideas. Some think that dark energy fills space with virtual particles that only briefly exist as matter and then disappear, while others are ready to entertain that Einstein's theory of gravity is incorrect, which could be a kind of game-changer as you'd imagine.

So the bottom line is, whatever dark energy is, whatever it's doing, it's doing it like crazy. But everybody wants to know more about dark energy so hopefully, we'll have more information soon.

Thank you for watching this SciShow Dose and thank you to all the people who suggested that we do an episode on dark energy. If you want to make other suggestions for us, or leave questions or comments or anything like that, we're on Facebook and Twitter, and of course down in the comments below, and if you want to keep getting smarter with us here at SciShow, you can go to youtube.com/scishow and subscribe.