Our Universe's 'Dark' Secret ¦ How the Universe Works ¦ Science Channel

Our home star, the Sun, is just one of 200 billion stars that make up our galaxy. But how did this vast collection of stars first form? View the Milky Way with imaginary dark matter goggles and you'll find a clue. A halo of dark matter surrounding the galaxy. A dark matter halo helps the galaxy form simply by providing gravity to pull things together, catalyzing it. And that may have allowed our galaxy to form. The gravity from this huge construct brought regular matter in to form the Milky Way in the middle. Astronomers used to think the distribution of galaxies throughout the universe was random, but recent observations have discovered something extraordinary. Walls of interlinked galaxies that stretch through space for millions and millions of light years. One of the most amazing discoveries of the last few decades is that galaxies form these vast superstructures that actually span the known universe. The Great Wall is a filament of galaxies that stretches hundreds of millions of light years. To understand how these vast galactic structures formed, astronomers used telescopes like the Atacama Large Millimeter Array, or ALMA for short. It's an array of 66 radio telescopes and so powerful, it can peer back billions of years to the formation of the earliest galactic structures. Now ALMA can actually look back into the history of the universe and see similar structures being formed by these giant baby galaxies, sort of a proto-Great Wall. ALMA gives us a snapshot of how the adolescent universe evolved. It shows us that as the universe expanded, the newborn galaxies aligned with their neighbors. It was as if the expanding universe was producing sticky filaments and the materials building new galaxies were sticking to these threads, like flies to a spider's silk. It turns out these invisible filaments are dark matter. It's like a scaffolding of dark matter that was pulling normal matter into it. As the universe expanded, the original clumps of dark matter didn't expand as quickly as the rest. They stuck together like sticky taffy with their powerful gravity shaping them into filaments. The filaments formed a sprawling web of dark matter strands stretching throughout the universe. The gravity of this dark matter web then dragged in normal matter, which built up where the filaments meet and eventually collapsed to form galaxies. The thicker filaments pulled in the most gas, providing the building blocks for galaxy clusters. They actually fall along these tremendous filaments across the universe, hundreds of millions of light years across. We're talking about tremendously large structures, but they would not exist if it weren't for dark matter. And the galaxies themselves were able to because of this structure. There are galaxies and stars and planets in you here today. That's because of the dark matter providing the framework. Just like a grid system in a city defines where buildings are going to be, galaxies assemble themselves around the cosmic grid. It seems like city planners here on Earth have been following the lead of the universe, except those planners used roads in place of dark matter. New York is the perfect example. Let's imagine New York City without roads. There would be no structure, no foundation, and the whole thing would fall apart. When people first designed the city, they laid down the grid, they built the roads, and that was really the foundation. And then later on, they built the buildings. Just like the dark matter web transported the building materials for galaxies, New York's grid of roads brought the steel and concrete to build its city blocks. It's the dark matter that gives you the foundation, it gives you the cosmic structure, and then later on, the normal matter fell into the galaxies and the clusters that we see today. The elegance of this newly discovered structure of the universe really astounds me. You have this web of dark matter, and it almost creates highways for regular matter to fall into these nexuses, and that's where you form the biggest, brightest galaxies. The more cosmologists study dark matter, the more they see the crucial role it's played in shaping the universe we see today. But despite these insights into our origins, scientists still don't know what dark matter actually is. How do you measure something that you can't see or feel? One of the properties of dark matter that we know for sure is that it doesn't interact with light. It doesn't shine, it doesn't reflect light, and so if you shine a laser beam on dark matter, the laser beam goes right through, it doesn't do anything. The term dark matter might actually be a little bit misleading. Maybe the real term for this stuff is transparent matter, but somehow that doesn't sound quite as cool as dark matter. So if we can't see dark matter, why are scientists so sure it's there? We know dark matter is there the same way we know many things are there that we can't see. We use gravity. Dark matter may not interact with light, but it interacts with gravity. It's only by studying the motions of stars, even the motions of galaxies, and the clustering of galaxies, do we know that there's a lot of extra stuff out there that has mass that's directing these stars and galaxies to move in ways that are somewhat unexpected. The first person to detect this unusual movement in galaxies was astronomer Fritz Zwicky in the 1930s. Using a custom-built 18-inch telescope, he studied how multiple galaxies interact inside a tight grouping of galaxies known as a cluster. Fritz Zwicky studied the motions of galaxies in clusters, and what he saw is that these galaxies were moving too fast. Every concentration of matter has associated with it an escape velocity, and if you're moving faster than the escape velocity, you should no longer be a part of that system. Just as a rocket can escape Earth's gravity if it's traveling fast enough, a galaxy should break away from a galaxy cluster if it's moving with enough speed. Zwicky noticed that the galaxies in the coma cluster should be moving fast enough to escape the gravitational pull of their neighbors. But instead of speeding off into different parts of the universe, the galaxies remained bound together. Zwicky thought some unknown material was adding mass and therefore extra gravity to the system, holding the galaxies in place. Maybe there's something extra there that's providing more gravity than we can account for based on what we see. At the time, Zwicky's ideas for this unexplained source of extra gravity fell on deaf ears. Zwicky was a very unlikable fellow, and I think infuriated many of his colleagues, and that perhaps was one other reason they were less willing to accept his suggestion. The other is that I think he was so far ahead of his time as a scientist that it just took time for the rest of the community to catch up.