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Have you ever had the feeling that there's something bigger out there, something you can't see?
你是否曾有过这样一种感觉：世界上存在一种更大却又看不见的东西？
Well there is – it's called Dark Matter.
有的！就是暗物质。
Dark matter is spread throughout space, engulfing the Milky Way and all other galaxies.
暗物质遍布太空，包围着银河系和所有其他星系。
It's four times more common in the universe than visible matter, yet we've never seen or directly observed it!
在宇宙中，它比可见物质多四倍，但我们从未看见过或直接观察到过它！
So how do we know it's there? Because of gravity.
所以我们是怎么知道它的存在呢？因为引力。
A galaxy is like a giant merry-go-round that all the stars are riding.
每个星系就像是一个巨大的旋转木马，上边骑着所有的恒星。
In order to stay on, each star has to hold on tight (that's where gravity comes in), otherwise it'll fly off into space.
为了坐稳，每颗恒星都必须牢牢抓紧（引力的作用就在此），否则它就会被甩进太空。
The problem is, the force of gravity from all the stars we can see isn't strong enough to hold galaxies together –
问题是，我们所能看到的所有恒星的引力不足以把星系聚集在一起—
all the stars should be flying off the edge of the merry-go-round! But they're not.
所有的恒心应该会被甩到旋转木马的边缘！但是它们没有。
It's as if some invisible rope is helping them hold on.
就好像有一根看不见的绳子帮助它们抓牢了一样。
Physicists believe that rope is the gravitational force from stuff we can't see - hence, Dark Matter.
物理学家们认为这根绳子是某种不可见物质产生的引力，即暗物质。
And how do we know the speeds of stars in distant galaxies?
我们如何知道遥远星系中恒星的速度呢？
The doppler effect (which is why a police siren shifts to a lower pitch as the car passes) also works for light –
多普勒效应（这就是为什么汽车经过时警笛的声音会变低）也适用于光线—
that's how radar catches you speeding, and how astronomers measure the rotation of galaxies.
正因为如此雷达才能抓住超速车辆，天文学家才能测量星系的旋转情况。
So, we use what we can see to tell us about what we can't.
所以我们能从已见中看见未见。
