1 00:00:00,000 --> 00:00:03,704 We all know what a galaxy looks like, right? It’s a huge 2 00:00:03,704 --> 00:00:06,106 collection of stars and other matter that’s shaped like a 3 00:00:06,106 --> 00:00:09,376 spiral or an ellipse, and if you’re an astronomy fan you 4 00:00:09,376 --> 00:00:11,945 probably know that most of the mass is from invisible, 5 00:00:11,945 --> 00:00:16,250 mysterious material called dark matter. Well, NASA’s Hubble 6 00:00:16,250 --> 00:00:20,020 Space Telescope just took an image of a galaxy that is none 7 00:00:20,020 --> 00:00:23,757 of those things. For the first time, astronomers have strong 8 00:00:23,757 --> 00:00:27,527 evidence for a galaxy not having a significant amount of dark 9 00:00:27,527 --> 00:00:31,031 matter. Most astronomers currently believe that dark 10 00:00:31,031 --> 00:00:34,368 matter plays a fundamental role in our universe and the 11 00:00:34,368 --> 00:00:37,204 formation of galaxies. This is because galaxies seem to have a 12 00:00:37,204 --> 00:00:40,340 lot more mass than what we can account for based on just the 13 00:00:40,340 --> 00:00:43,644 stars we see. How much mass is in a system is determined by 14 00:00:43,644 --> 00:00:46,313 measuring the speed at which galaxies rotate or individual 15 00:00:46,313 --> 00:00:50,484 stars in a galaxy move. Without the gravity from that mass, a 16 00:00:50,484 --> 00:00:53,186 galaxy would fly apart if everything in it is moving as 17 00:00:53,186 --> 00:00:57,157 quickly as we observe it moving. Decades of research have led 18 00:00:57,157 --> 00:01:01,495 astronomers to the extraordinary conclusion that 85% of the mass 19 00:01:01,495 --> 00:01:05,666 in our universe is invisible in all wavelengths of radiation, 20 00:01:05,666 --> 00:01:08,602 and that it’s composed of matter that does not contain protons or 21 00:01:08,602 --> 00:01:12,406 neutrons or any type of particle we’ve detected before, AND that 22 00:01:12,406 --> 00:01:16,043 this invisible material is all around us, passing through us 23 00:01:16,043 --> 00:01:20,247 without interacting with regular matter except by gravity. For 24 00:01:20,247 --> 00:01:23,050 some, that’s a tough pill to swallow, and a minority of 25 00:01:23,050 --> 00:01:25,919 astronomers wonder if maybe we just don’t completely understand 26 00:01:25,919 --> 00:01:30,190 how gravity works. If that were the case, and it was an inherent 27 00:01:30,190 --> 00:01:32,926 property of gravity that causes galaxies to move the way they 28 00:01:32,926 --> 00:01:36,997 do, then we could expect all galaxies to behave the same way. 29 00:01:36,997 --> 00:01:39,599 In other words, they would all seem to have about the same 30 00:01:39,599 --> 00:01:43,403 portion of “dark matter.” But with the galaxy in this Hubble 31 00:01:43,403 --> 00:01:46,540 image, astronomers looked at the velocities of ten globular 32 00:01:46,540 --> 00:01:49,676 clusters in the galaxy, each a spherical collection of hundreds 33 00:01:49,676 --> 00:01:52,679 of thousands of stars, and calculated that their movements 34 00:01:52,679 --> 00:01:56,183 can be accounted for entirely by the mass of the visible material 35 00:01:56,183 --> 00:02:00,153 in this system. That means this galaxy has little to no dark 36 00:02:00,153 --> 00:02:04,257 matter. Strangely, this absence of dark matter actually provides 37 00:02:04,257 --> 00:02:07,394 evidence that dark matter is real. It shows that dark matter 38 00:02:07,394 --> 00:02:09,663 isn’t always coupled with regular matter – that it’s 39 00:02:09,663 --> 00:02:13,500 something separate. You can have regular matter without dark 40 00:02:13,500 --> 00:02:16,803 matter. This galaxy is really weird even beyond the dark 41 00:02:16,803 --> 00:02:19,473 matter thing. You may have noticed you can see straight 42 00:02:19,473 --> 00:02:22,609 through it. That’s because this galaxy is what’s called an 43 00:02:22,609 --> 00:02:26,113 “ultra diffuse galaxy,” which as the name implies, is extremely 44 00:02:26,113 --> 00:02:29,549 low density. This galaxy is about the same volume as our own 45 00:02:29,549 --> 00:02:34,921 Milky Way galaxy, but only has about 0.5% the amount of stars. 46 00:02:34,921 --> 00:02:37,190 Though astronomers have known about ultra diffuse galaxies 47 00:02:37,190 --> 00:02:40,227 since the early 1980s, they can be difficult to find since 48 00:02:40,227 --> 00:02:43,630 they’re so faint. A team of astronomers is using an array of 49 00:02:43,630 --> 00:02:46,633 telephoto lenses called Dragonfly to seek out these 50 00:02:46,633 --> 00:02:49,770 ghostly-looking objects. They obtained observations from 51 00:02:49,770 --> 00:02:53,640 Dragonfly, the Sloan Digital Sky Survey, the Gemini Observatory, 52 00:02:53,640 --> 00:02:56,610 and the Keck Observatory, then requested time on the Hubble 53 00:02:56,610 --> 00:03:00,514 Space Telescope to take a closer look at this unusual galaxy. 54 00:03:00,514 --> 00:03:03,116 Having images and data from multiple sources allowed the 55 00:03:03,116 --> 00:03:05,686 team to determine that this galaxy does not have a 56 00:03:05,686 --> 00:03:08,922 significant amount of dark matter. This was definitely 57 00:03:08,922 --> 00:03:12,492 surprising to find. No other galaxies so far have appeared to 58 00:03:12,492 --> 00:03:16,396 be so lacking in dark matter. In fact, other ultra diffuse 59 00:03:16,396 --> 00:03:19,332 galaxies seem to have an overabundance of dark matter. 60 00:03:19,332 --> 00:03:22,536 The same team who studied this galaxy discovered a different 61 00:03:22,536 --> 00:03:28,075 ultra diffuse galaxy in 2016 that they calculated was 99.9% 62 00:03:28,075 --> 00:03:32,245 dark matter. Yet another weird thing about this galaxy – the 63 00:03:32,245 --> 00:03:35,048 globular clusters used to measure the galaxy’s rotation 64 00:03:35,048 --> 00:03:38,518 are way brighter than normal globular clusters. The 65 00:03:38,518 --> 00:03:40,787 researchers have written a different paper that focuses on 66 00:03:40,787 --> 00:03:44,925 just these oddball collections of stars. So, this is a very 67 00:03:44,925 --> 00:03:48,395 strange galaxy in several ways. Astronomers will be looking at 68 00:03:48,395 --> 00:03:51,565 Hubble observations of other ultra diffuse galaxies to see if 69 00:03:51,565 --> 00:03:54,534 there are any other examples of galaxies with unusually low or 70 00:03:54,534 --> 00:03:57,838 high amounts of dark matter. With more samples, astronomers 71 00:03:57,838 --> 00:03:59,906 will be able to better understand the nature of dark 72 00:03:59,906 --> 00:04:03,076 matter, the formation and evolution of galaxies, and the 73 00:04:03,076 --> 00:04:05,712 overall structure of our universe. 74 00:04:05,712 --> 00:00:00,000 www.nasa.gov/hubble @NASAHubble