1 00:00:02,300 --> 00:00:04,500 Many of us follow the idea of "seeing is believing" 2 00:00:04,600 --> 00:00:07,500 But we all know that there are a lot of times 3 00:00:07,600 --> 00:00:09,200 where this isn't exactly true. 4 00:00:09,300 --> 00:00:10,400 One of the most mysterious of which 5 00:00:10,500 --> 00:00:13,000 could be key to understanding the universe around us 6 00:00:15,100 --> 00:00:17,600 Dark matter is defined as particles that don't 7 00:00:17,700 --> 00:00:20,100 absorb, reflect, or emit light 8 00:00:20,200 --> 00:00:22,000 They don't really interact with normal matter, 9 00:00:22,100 --> 00:00:25,100 and they can't be detected using electromagnetic radiation. 10 00:00:25,400 --> 00:00:27,500 Essentially, with current technology as it is, 11 00:00:27,600 --> 00:00:30,900 we have no way of really finding dark matter in space 12 00:00:31,000 --> 00:00:33,100 But if we can't detect dark matter... 13 00:00:33,200 --> 00:00:34,200 how do we know it's there? 14 00:00:34,900 --> 00:00:36,800 To make it easier to visualize, let's look at 15 00:00:36,900 --> 00:00:38,700 two bottles with coins in them 16 00:00:38,800 --> 00:00:40,500 One of these bottles is full of water, 17 00:00:40,600 --> 00:00:41,900 while the other has just air. 18 00:00:42,000 --> 00:00:44,300 You really can't tell the difference between 19 00:00:44,400 --> 00:00:46,400 the bottles just by looking at them. What you 20 00:00:46,500 --> 00:00:48,500 can see however is that the water makes the coins 21 00:00:48,600 --> 00:00:49,800 move differently than air 22 00:00:49,900 --> 00:00:53,100 Let's abstract this thinking into how dark matter was discovered. 23 00:00:53,200 --> 00:00:55,800 The first person to discover and coin the term 24 00:00:55,900 --> 00:00:58,600 "dark matter" was an astronomer named Fritz Zwicky. 25 00:00:58,700 --> 00:01:01,700 In the 1930s, he was observing the Coma galaxy cluster, 26 00:01:01,800 --> 00:01:03,900 and he used a method called the "virial theorem" 27 00:01:04,000 --> 00:01:06,600 to infer the gravitational mass of the cluster. 28 00:01:06,700 --> 00:01:09,500 His results led him to argue that there was simply not enough 29 00:01:09,600 --> 00:01:11,700 combined mass of visual matter in the cluster 30 00:01:11,800 --> 00:01:13,300 to hold it together gravitationally 31 00:01:13,900 --> 00:01:16,500 Zwicky didn't have enough evidence to prove the existence 32 00:01:16,600 --> 00:01:19,700 of dark matter, but more evidence of this mysterious substance 33 00:01:19,800 --> 00:01:23,000 has popped up over the years. Let's look at one more example. 34 00:01:23,600 --> 00:01:25,700 When you look at the image of a galaxy, you can see 35 00:01:25,800 --> 00:01:28,400 that there are more stars concentrated near the center 36 00:01:28,500 --> 00:01:30,500 of the galaxy rather than the edges. 37 00:01:30,600 --> 00:01:33,100 This leads astronomers to the logical conclusion that the 38 00:01:33,200 --> 00:01:35,400 gravity must be stronger near the center and weaker 39 00:01:35,500 --> 00:01:37,400 near the edges of the galaxy. 40 00:01:37,500 --> 00:01:39,100 This means that the stars near the edges of the 41 00:01:39,200 --> 00:01:42,000 galaxy have to orbit slower than the ones near the center. 42 00:01:42,100 --> 00:01:44,400 Otherwise, they'd break the force of gravity and 43 00:01:44,500 --> 00:01:45,800 move off into space 44 00:01:45,900 --> 00:01:49,300 While observing the Andromeda galaxy, astronomer Vera Rubin 45 00:01:49,400 --> 00:01:52,800 was testing out the new spectrometer of a colleague, Kent Ford Jr. 46 00:01:52,900 --> 00:01:55,200 The wavelengths measured using the spectrometer 47 00:01:55,300 --> 00:01:58,100 allowed them to calculate the speed of the stars in the galaxy. 48 00:01:58,200 --> 00:02:00,700 What they found, however, was very unexpected. 49 00:02:00,800 --> 00:02:03,800 All of the stars in the galaxy were orbiting at the same speed! 50 00:02:03,900 --> 00:02:07,400 But there simply wasn't enough visible mass in the galaxy 51 00:02:07,500 --> 00:02:11,000 to create gravity strong enough to keep those stars in orbit. 52 00:02:11,700 --> 00:02:15,900 Unless there was something else, in between the stars - Dark matter. 53 00:02:16,300 --> 00:02:19,200 But what is dark matter exactly? 54 00:02:19,300 --> 00:02:21,000 Because of its properties, we currently have no 55 00:02:21,100 --> 00:02:23,300 way to find out exactly what dark matter consists of. 56 00:02:23,700 --> 00:02:27,600 There have been many candidates - WIMPs, axions, bosons, 57 00:02:27,700 --> 00:02:30,300 and neutrinos being just a few, but with many of 58 00:02:30,400 --> 00:02:33,500 these particles being theoretical or some of the most elusive known to 59 00:02:33,600 --> 00:02:37,300 man, hopes have been constantly raised and dashed with no results. 60 00:02:37,400 --> 00:02:40,500 Rise of new technology brings rise of new information, 61 00:02:40,600 --> 00:02:43,100 and along with it comes ways of finding out how the universe 62 00:02:43,200 --> 00:02:45,600 around us came to be. Dark matter may have 63 00:02:45,700 --> 00:02:48,300 existed even before the Big Bang, which gives us insight 64 00:02:48,400 --> 00:02:50,900 not only into the Big Bang and how matter came into the 65 00:02:51,000 --> 00:02:53,200 arrangements that it's in today, but also about times 66 00:02:53,300 --> 00:02:55,300 even before the big bang and the known universe. 67 00:02:55,500 --> 00:02:57,500 And with that, I'll leave you to think about 68 00:02:57,600 --> 00:02:59,600 what else lurks in the dark.