Of all the spectacles mankind has viewed through a telescope, there are few lovelier than a spiral galaxy.
人类通过望远镜观测到的所有景色中，几乎没有什么比螺旋星系更可爱。
Majestic whirlpools of stars, they rotate in a stately and predictable dance.
那是恒星雄伟的旋涡，一种壮观而可预测的舞蹈。
The fact that we see many billions of them in our telescopes tells us they are both common and stable.
我们通过望远镜能看到几十亿恒星，这个事实告诉我们，它们既常见又稳定。
It is perhaps surprising that it is relatively easy to understand the inner workings of these cosmic pinwheels.
也许令人惊讶，但理解这些宇宙风车的内部运作并不算难。
By combining physical principles worked out by Sir Isaac Newton in the late 17th century,
通过结合艾萨克·牛顿爵士在17世纪末发现的物理原理
with the observed amount of mass in a galaxy, scientists can calculate the rates at which these galaxies rotate.
和观测到的星系质量，科学家可以算出星系旋转的速率。
Using these techniques, astronomers predict how fast stars at different distances from the center of the galaxy should move.
天文学家用这些技术预测了与中心距离不同的恒星的运动速度。
Stars very close to the center move slowly.
中心附近的恒星移动缓慢。
That's because there is very little mass between them and the center of the galaxy to pull them along.
这是因为，它们和星系中心之间的质量太少，不能有力地牵引。
Stars a bit further away move faster, because they are being pulled by all of the stars in between them and the center.
稍远的恒星运动速度快一些，因为它们受到了和中心之间所有星星的牵引。

As we get really far away, the stars are predicted to move slowly again.
离中心很远的地方，恒星运动应该要再次慢下来。
Their great distance reduces gravity to a gentle tug, so they move leisurely in their orbits.
漫长的距离使引力减弱成了轻轻一拽，因此它们在轨道上悠闲地前进。
Knowing this, scientists looked at the galaxies and measured how fast stars were moving.
了解这些后，科学家观察了星系中恒星的移动速度。
To their surprise, they found that while the stars closer to the center of the galaxy behaved as predicted,
他们惊讶地发现，星系中心附近恒星的运动符合预测，
those further away moved far too quickly.
但更远处的星星移动地有点太快了。
This observation was devastating to the tradtional theories of gravity and motion.
这个发现挑战了重力和运动的传统理论。
If the stars were moving as fast as their measurement suggested, galaxies should have torn themselves apart.
如果这些恒星移动得和测量结果一样快，那星系早就该把自己撕碎了。
It was a crisis, and astronomers and physicists scrambled to find a mistake in their calculation.
这是一场危机，天文学家和物理学家纷纷寻找演算中的错误。
Was Newton's theory of gravity wrong? Was his theory of motion wrong?
是牛顿的万有引力定律错了吗？还是他的运动定律错了？
Or was it possible that astronomers had incorrectly measured the galaxy's mass?
有没有可能是天文学家错误地测量了星系的质量？
All options were investigated, and all were ruled out. Except one.
经过研究，所有可能都被排除了。除了一个。
Today, scientists believe that the answer lies in a previously unknown kind of matter, called dark matter.
今天，科学家相信，答案就在一种之前不为人所知的物质中：暗物质。
This dark matter can be envisioned as a cloud which surrounds most galaxies. This matter is very unusual.
暗物质就像云，充满大部分星系。这种物质很特别。
It is affected by gravity, but it is invisible to visible light and all other forms of electromagnetic radiation.
它受到引力的影响，但对可见光和其他所有电磁辐射都不可见。
The name 'dark matter' originates in this form of matter's inability to emit or absorb light.
“暗物质”叫这个名字，就是因为它既不反射也不吸收光。
Dark matter adds to the gravity of the galaxy and explains the orbital speed of stars far from the galactic center.
暗物质增加了星系的引力，也解释了星系边缘恒星的轨道速度。
Dark matter has not yet been directly observed, but scientists believe that it is likely to be real,
暗物质至今未被直接观测到，但科学家相信极有可能确有其事，
mostly because the other options have been ruled out.
主要是因为其他可能都被排除了。
Using dozens of approaches, astronomers and physicists continue to search for direct evidence
天文学家和物理学家想了很多办法寻找直接证据
that would prove that the dark matter hypothesis is true.
来证实暗物质假设。
This question is one of the most important physics research questions of the 21st century.
这个问题是21世纪物理学研究中最重要的问题之一。