1 00:00:11,010 --> 00:00:06,970 Music 2 00:00:11,030 --> 00:00:15,050 For the past 40 years, astronomers have known that something about the 3 00:00:15,070 --> 00:00:19,090 cosmos doesn't add up. First in galaxy clusters 4 00:00:19,110 --> 00:00:23,130 and then within individual galaxies, they found that visible matter 5 00:00:23,150 --> 00:00:27,180 --stars, gas and dust--cannot account for motions they observe. 6 00:00:27,200 --> 00:00:31,260 No one knows what this missing mass, now 7 00:00:31,280 --> 00:00:35,280 called "dark matter," actually is, but studies by NASA's 8 00:00:35,300 --> 00:00:39,290 WMPA spacecraft of the cosmic microwave background--the oldest 9 00:00:39,310 --> 00:00:43,370 light in the universe--show how much is out there. Dark 10 00:00:43,390 --> 00:00:47,380 matter outnumbers ordinary matter by 4 to 1. 11 00:00:47,400 --> 00:00:51,410 The WMAP results also hint that dark matter likely takes the form on an 12 00:00:51,430 --> 00:00:55,420 as-yet-undiscovered subatomic particle. WIMPs 13 00:00:55,440 --> 00:00:59,440 represent one hypothesized class of these particles. They 14 00:00:59,460 --> 00:01:03,470 neither absorb nor emit light, and don't interact strongly with other particles. 15 00:01:03,490 --> 00:01:07,500 But when they encounter each other, they annihilate and make gamma rays. 16 00:01:07,520 --> 00:01:11,540 That's where NASA's Fermi Gamma-ray Space 17 00:01:11,560 --> 00:01:15,560 Telescope comes in. Two years of scanning the sky with Fermi's 18 00:01:15,580 --> 00:01:19,600 Large Area Telescope have set the strongest limits yet for WIMP dark 19 00:01:19,620 --> 00:01:23,630 matter. The best place to look for gamma rays from dark matter annihilation? 20 00:01:23,650 --> 00:01:27,660 The most boring galaxies around, called dwarf spheroidals. 21 00:01:27,680 --> 00:01:31,690 These faint, tiny galaxies possess impressive 22 00:01:31,710 --> 00:01:35,770 amounts of dark matter, but they contain no gamma-ray-emitting objects, 23 00:01:35,790 --> 00:01:39,820 and little gas or star formation. In the currently accepted 24 00:01:39,840 --> 00:01:43,890 cosmology, the first structures formed as the gravitation of dark matter 25 00:01:43,910 --> 00:01:47,970 corralled normal matter. Simulations show that the largest structures 26 00:01:47,990 --> 00:01:52,090 formed in this way were comparable to the dwarf spheroidal galaxies we see 27 00:01:52,110 --> 00:01:56,120 today. It's thought that large galaxies like our own were 28 00:01:56,140 --> 00:02:00,150 built-up from collisions among these dwarfs. 29 00:02:00,170 --> 00:02:04,170 Using two years of data, Fermi scientists explored ten dwarf 30 00:02:04,190 --> 00:02:08,250 galaxies for an sign of gamma rays from WIMP annihilation. In 31 00:02:08,270 --> 00:02:12,330 this graph, the dashed line marks the sweet spot where conventional expectations 32 00:02:12,350 --> 00:02:16,360 for WIMP dark matter align with what we know about our universe. 33 00:02:16,380 --> 00:02:20,390 Even when scientists combine all of the Fermi data from all ten of the 34 00:02:20,410 --> 00:02:24,440 dwarfs, they see no sign of gamma rays. This limit shrinks 35 00:02:24,460 --> 00:02:28,460 the box where WIMP-based dark matter may be found, and for the 36 00:02:28,480 --> 00:02:32,500 first time, shows that the cosmology we know essentially eliminates some 37 00:02:32,520 --> 00:02:36,600 WIMP types. The longer Fermi operates, the better 38 00:02:36,620 --> 00:02:40,650 its ability either to box in the nature of dark matter, or to find actual 39 00:02:40,670 --> 00:02:44,690 evidence of what it is. And the discovery new dwarf galaxies will 40 00:02:44,710 --> 00:02:48,760 make this search even more sensitive. Although 41 00:02:48,780 --> 00:02:52,840 nondescript, dwarf spheroidal galaxies may have been the first large 42 00:02:52,860 --> 00:02:56,910 structures to form in the universe. Now, they've taken 43 00:02:56,930 --> 00:03:00,930 center stage in the drama to solve one of astronomy's greatest mysteries. 44 00:03:00,950 --> 00:03:04,960 Humm and beep