0:00 Okay, here’s a number for you.0:02 Less than five percent.0:03 Everything you've ever seen—every star, every planet, every grain of sand, you, me—all of it makes up less than five percent of the known universe.0:14 It’s an incredible thought, isn’t it?0:17 Last week we talked about the universe’s tendency toward chaos, like shuffling a deck of cards.0:24 But this is a whole other level of mystery.0:28 It’s not about the order of the cards; it’s about the fact that most of the deck is invisible.0:35 Exactly!0:35 So my big question is...0:37 what IS the other ninety-five percent?0:40 What are we missing?0:41 Well, scientists have split that giant question mark into two parts.0:46 The first is called Dark Matter, and the second is called Dark Energy.0:52 Dark Matter.0:52 That sounds very dramatic.0:54 What does it do?0:56 Okay, so think of a galaxy, like our own Milky Way.0:59 It’s spinning, right?1:01 And you'd expect the stars on the far, far edges to be spinning slower than the stars near the center.1:09 Yeah, like a merry-go-round.1:11 The horses on the outside don't have to move as fast.1:15 Uh, actually the opposite.1:17 The horses on the outside have to go WAAAY faster to keep up.1:21 But in a galaxy, gravity is what holds it all together.1:25 So the stars on the edge, where the gravity from all the visible stuff is weaker, should be moving slower.1:33 If they moved too fast, they’d just...1:36 fly off into space.1:37 Okay, that makes sense.1:39 So they’re moving slower.1:41 But they aren't!1:42 That’s the whole mystery.1:44 When astronomers measured them in the 1970s, a scientist named Vera Rubin found that the stars on the edge were moving just as fast as the ones closer in.1:56 It broke the rules.1:57 Whoa.1:57 So...1:58 what’s stopping them from flying away?2:01 Exactly!2:01 There must be a huge amount of extra mass in that galaxy that we can't see, providing the extra gravitational glue to hold those speedy stars in place.2:13 That invisible mass… that's dark matter.2:16 It has gravity, but it doesn’t reflect light, or block light, or give off light.2:21 It’s just… dark.2:23 So it’s not just, like, dust clouds or a bunch of black holes?2:27 Scientists have checked!2:29 They’ve done the math, and all the normal stuff we know—even the dark, non-glowing normal stuff—doesn't add up to nearly enough.2:39 It has to be something completely new.2:42 Some kind of particle that zips right through us without a trace.2:46 Okay, so that’s Dark Matter.2:48 That’s about twenty-seven percent of the universe, you said.2:53 What about the rest?2:54 The biggest piece of the pie?2:57 Ah.2:57 Now we get to Dark Energy.2:59 And honestly, this might be the single strangest discovery in all of science.3:05 Stranger than invisible gravity glue?3:07 Oh yeah.3:08 So, for most of the twentieth century, we knew the universe was expanding since the Big Bang.3:15 And the big question was, will it expand forever, or will gravity eventually pull everything back together in a "Big Crunch"?3:24 Like throwing a ball up in the air.3:27 Gravity is always trying to pull it back down.3:30 Precisely.3:31 So in the late 1990s, two teams of astronomers set out to measure the rate of that slowdown.3:38 They used distant supernovae—these incredibly bright exploding stars—as cosmic mile-markers.3:45 And they found the opposite of what they expected.3:48 The expansion of the universe isn't slowing down at all.3:53 Wait—what?3:53 It's speeding up.3:55 Hold on.3:55 How?3:56 What could be powerful enough to push the entire universe apart against the pull of gravity?4:02 We have absolutely no idea.4:04 That’s the honest answer.4:06 There is some kind of energy, some kind of pressure woven into the fabric of space-time itself that is acting like anti-gravity.4:16 We gave that mysterious force a name: Dark Energy.4:20 And it makes up about sixty-eight percent of everything.4:24 So the universe is being held together by invisible matter while it’s simultaneously being pushed apart by an invisible energy.4:33 You got it.4:34 We are living in the five percent.4:37 We’re just the foam on top of this deep, dark, mysterious ocean.4:41 It means the biggest, most fundamental questions are still completely unanswered.4:48 And all of our physics, everything we think we know, is based on that tiny little slice of reality we can actually see.4:56 Yep.4:57 It’s a pretty good reminder that we don't get to tell the universe how it should work.5:03 We just get the amazing job of trying to listen to what it's telling us.
