there are few lovelier than a spiral galaxy.

螺旋星系是最美好的奇景之一

Majestic whirlpools of stars, they rotate in a stately and predictable dance.

星群形成壯麗漩渦， 在其中跳著穩定恆常的舞步

The fact that we see many billions of them in our telescopes

用望遠鏡便可觀察到 幾十億個以上的螺旋星系

tells us they are both common and stable.

可見得它既普遍、又穩定

It is perhaps surprising that it is relatively easy to understand the inner workings of these cosmic pinwheels.

可能會嚇你一跳，要了解這些 宇宙風車的內部運作

By combining physical principles worked out by Sir Isaac Newton in the late 17th century,

最容易的是依據牛頓 (Sir Isaac Newton) 在十七世紀晚期研究出的物理原則

with the observed amount of mass in a galaxy,

以及觀測到的星系質量

scientists can calculate the rates at which these galaxies rotate.

科學家便可計算出星系旋轉速率

Using these techniques, astronomers predict how fast stars at different distances from the center of the galaxy should move.

天文學家用此法預測與中心 不同距離的恆星移動的速度

Stars very close to the center move slowly.

越靠近星系中心，恆星移動越慢

That's because there is very little mass between them and the center of the galaxy to pull them along.

因為它們之間沒有很多質量可以甩動

Stars a bit further away move faster,

遠一點的恆星移動速度較快

because they are being pulled by all of the stars in between them and the center.

因為它們被它們與核心之中的所有星星 一起甩動

As we get really far away, the stars are predicted to move slowly again.

到很遠以後， 恆星的移動速度又會慢下來

Their great distance reduces gravity to a gentle tug,

因為距離太遠，重力變得微弱

so they move leisurely in their orbits.

因此就悠閒的在軌道漫步

Knowing this, scientists looked at the galaxies and measured how fast stars were moving.

科學家觀測星系中 各恆星的軌道速度

To their surprise, they found that while the stars closer to the center of the galaxy behaved as predicted,

驚訝的發現，接近星系中心的恆星 速度和預測相符

those further away moved far too quickly.

但較遠的恆星，速度超出預期

This observation was devastating to the tradtional theories of gravity and motion.

這對傳統的重力及運動理論 帶來毀滅性打擊

If the stars were moving as fast as their measurement suggested,

如果恆星移動的速度和遠近符合定律的話

galaxies should have torn themselves apart.

星系早就被自己撕裂了。

It was a crisis, and astronomers and physicists scrambled to find a mistake in their calculation.

這是個危機，天文學家和物理學家 拼命想找出哪裡算錯了

Was Newton's theory of gravity wrong?

是牛頓的萬有引力定律錯了嗎？

Was his theory of motion wrong?

還是他的三大運動定律錯了嗎？

Or was it possible that astronomers had incorrectly measured the galaxy's mass?

或是天文學家算錯星系的質量呢？

All options were investigated, and all were ruled out. Except one.

調查並排除所有選項後 只剩下一種可能

Today, scientists believe that the answer lies in a previously unknown kind of matter, called dark matter.

如今科學家相信，答案就是 一種未知的物質，稱為「暗物質」

This dark matter can be envisioned as a cloud which surrounds most galaxies.

暗物質好像一團雲 籠罩了多數的星系

This matter is very unusual. It is affected by gravity,

這是很不尋常的，它受到重力影響

but it is invisible to visible light and all other forms of electromagnetic radiation.

但是看不見，也沒有 任何形式的電磁輻射

The name "dark matter" originates in this form of matter's inability to emit or absorb light.

「暗物質」名稱源自這種物質 不會發出或吸收光線

Dark matter adds to the gravity of the galaxy

暗物質增加了星系的重力

and explains the orbital speed of stars far from the galactic center.

並可解釋遠離星系中心 恆星的軌道速度

Dark matter has not yet been directly observed,

暗物質從未被直接觀測到

but scientists believe that it is likely to be real,

但科學家相信，它很可能是真的

mostly because the other options have been ruled out.

主要因為其他可能性都已經排除

Using dozens of approaches, astronomers and physicists continue to search for direct evidence

天文學家和物理學家正在嘗試 幾十種方法，想找出直接證據

that would prove that the dark matter hypothesis is true.

要證明，暗物質的假設是正確的

This question is one of the most important physics research questions of the 21st century.

這是 21 世紀物理學 最重要的研究課題之一