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Of all the spectacles mankind has viewed through a telescope,
人類由望遠鏡觀察到的宇宙中
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there are few lovelier than a spiral galaxy.
螺旋星系是最美好的奇景之一
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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
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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
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tells us they are both common and stable.
可見得它既普遍、又穩定
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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
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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
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with the observed amount of mass in a galaxy,
以及觀測到的星系質量
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scientists can calculate the rates at which these galaxies rotate.
科學家便可計算出星系旋轉速率
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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
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Stars very close to the center move slowly.
越靠近星系中心，恆星移動越慢
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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
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Stars a bit further away move faster,
遠一點的恆星移動速度較快
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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
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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
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Their great distance reduces gravity to a gentle tug,
因為距離太遠，重力變得微弱
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so they move leisurely in their orbits.
因此就悠閒的在軌道漫步
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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
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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
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those further away moved far too quickly.
但較遠的恆星，速度超出預期
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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
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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
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galaxies should have torn themselves apart.
星系早就被自己撕裂了。
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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
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Was Newton's theory of gravity wrong?
是牛頓的萬有引力定律錯了嗎？
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Was his theory of motion wrong?
還是他的三大運動定律錯了嗎？
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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？
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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
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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」
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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
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This matter is very unusual. It is affected by gravity,
這是很不尋常的，它受到重力影響
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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
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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
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Dark matter adds to the gravity of the galaxy
暗物質增加了星系的重力
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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
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Dark matter has not yet been directly observed,
暗物質從未被直接觀測到
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but scientists believe that it is likely to be real,
但科學家相信，它很可能是真的
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mostly because the other options have been ruled out.
主要因為其他可能性都已經排除
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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
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that would prove that the dark matter hypothesis is true.
要證明，暗物質的假設是正確的
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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
