Well, there is. It's called Dark Matter.

事實上是有的，它叫「暗物質」。

Dark matter is spread throughout space, engulfing the Milky Way and all other galaxies.

暗物質在宇宙間無所不在，包圍著銀河系和其他所有星系。

It's four times more common in the universe than visible matter, yet we've never seen or directly observed it!

暗物質是宇宙中可見物質的四倍之多，但我們卻無法看見或直接觀察它！

So how do we know it's there? Because of gravity.

那我們如何知道它的存在？因為引力。

A galaxy is like a giant merry-go-round that all the stars are riding.

星系就像個大型旋轉木馬，上面承載著所有恆星。

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.

為了坐穩，每顆恆星都得抓緊——這就是引力來源——否則它就會被甩進太空。

The problem is, the force of gravity from all the stars we can see isn't strong enough to hold galaxies together.

問題是，所有可觀測的恆星所產生之引力都不夠強，無法將星系維繫在一起。

All the stars should be flying off the edge of the merry-go-round. But they're not.

換言之，所有的恆星早就該被甩下旋轉木馬了！

It's as if some invisible rope is helping them hold on.

但它們沒有，就好像有條隱形的繩索在支撐它們一樣。

Physicists believe that rope is the gravitational force from stuff we can't see, hence, Dark Matter.

物理學家們相信，那條「繩子」是某種不可見物質所產生的引力，也就是「暗物質」。

And how do we know the speeds of stars in distant galaxies?

那我們如何知道遙遠星系中，恆星的速度呢？

The doppler effect, which is why a police siren shifts to a lower pitch as the car passes, also works for light.

都卜勒效應，就是警車經過後，警笛聲調降低的原理，會告訴我們，這原理也可運用於光線。

That's how radar catches you speeding, and how astronomers measure the rotation of galaxies.

正因如此，雷達才能抓到超速車輛、天文學家才能測量星系的運轉情況。

So, we use what we can see to tell us about what we can't.

所以，我們可以利用冰山一角窺視宇宙全貌（以「可見」推論「不可見」）。