You’ve learned all about atoms in science class. Now you think you’ve got a pretty good idea of what the universe is made of. Think again. The universe, it seems, is a lot weirder2 than meets the eye. Scientists believe that the stuff we know as matter makes up only a small fraction3 of the universe. The rest is invisible4 stuff so strange that no one, not even the brightest minds know what it is.
“For the longest time, we thought that atoms were all there was to the universe—atoms of regular matter,” astronomer Andrew Fraknoi, of Foothill College, told Current Science. “But in fact, it’s now appearing that that’s not the whole story—that there is a great deal of ‘something else.’ And that ‘something else’ is one of the great unsolved5 mysteries of the universe.”
Just 4 percent of the universe is ordinary matter, according to the most recent calculations. Twenty-three percent is a mysterious, invisible form of matter called dark matter. And a whopping6 73 percent is something else so poorly understood that scientists simply call it dark energy.
“You can imagine how frustrating7 it is for astronomers, whose job it is to know what’s in the universe, to have to get up in the morning, look in the mirror, and say, ‘I don’t know what most of the universe is made of,’” says Fraknoi.
Spin8 Rate?
If dark matter can’t be seen, how does anyone know it’s there? The tip-off9 came decades ago when astronomers noticed something very odd about the motion of the galaxies10. They spin way11 too fast—impossibly fast.
Galaxies are collections of millions or even billions of stars held together by gravity as they rotate12. Gravity is an attractive force that pulls together all things made of matter. The more matter there is, the stronger the force of gravity. When astronomers took careful stock of13 all the matter in the galaxies, they found that there wasn’t enough of it to produce the gravity necessary to keep the spinning galaxies from flying apart.
That finding is also true of our own galaxy, the Milky Way. “When we add up all the stuff we can see in the Milky Way14, there just isn’t enough gravity in all the visible material of the Milky Way to hold this fast-moving stuff together,” explains Fraknoi. “So we conclude from this that there must be more gravity than meets the eye.”
Astronomers think that the extra gravity holding galaxies together comes from matter that can’t be seen or detected—dark matter. To prove the existence of dark matter, however, scientists have to find it. And finding it is tricky15 because no one knows what to look for.
Some think dark matter might be ordinary matter trapped in hard-to-see places—perhaps in black holes or brown dwarfs. Others think it might be an exotic kind of matter that surrounds us without our even noticing it and passes through ordinary matter undetected.
Although dark matter remains elusive, there’s still good reason to believe it exists. Its gravity is strong enough to bend light, a phenomenon called gravitational lensing. Using powerful telescopes16, astronomers can see the telltale distortion caused by dark matter bending light that passes through or near it.
Universe Expanding
Not long ago, astronomers noticed something else strange about the universe. It is not only expanding—the rate of its expansion is actually speeding up.
The expansion of the universe can be explained by the big bang, the idea that 13.7 billion years ago, the universe exploded outward from a single point. “Space is stretching,” Fraknoi says, “like the skin of a balloon stretches when you blow it up.”
The universe’s expansion should be slowing down because of gravity. Instead, it’s speeding up—an occurrence so bizarre17 it’s as if you tossed a ball in the air and instead of coming back down, it flew faster and faster upward.
“If the universe is speeding up,” says Fraknoi, “there has to be another form of energy, something very powerful that is speeding up the universe.” Astronomers have named that other form of energy dark energy. Except for the fact that it seems to be a kind of antigravity force that pushes matter apart, no one has any idea what it might be.
Figuring out what dark energy is, how much there is, and how it affects the universe is no small matter. The answer could also settle an ongoing18 debate about the ultimate19 fate of the universe. Will it keep expanding forever—a trend known as the big rip20? Or will it eventually slow down, reverse course, and collapse—the big crunch21?
Fraknoi finds the puzzles of dark matter and dark energy tremendously exciting. “In astronomy, it’s pretty clear that there are far more things that we don’t know than we do know,” he says. “But if everything was answered, who’d become a scientist?”
在自然科学课上，你们对原子已经有了全面的了解。于是你们自认为对宇宙的构成有一个清晰的概念。但你们需要重新考虑这个问题，因为宇宙似乎远比它呈现在我们眼前的更玄妙莫测。
科学家们相信，我们称之为物质的东西仅占宇宙的一小部分。其余的部分是隐形的，这些东西如此奇特，以至于无人知道它为何物，连智商最高的人也无从知道。
“长期以来，我们一直认为原子——常规物质的原子，是构成宇宙的全部，”山麓学院的天文学家安德鲁·弗拉克诺伊对《今日科学》说，“但事实上，现在看起来这种认识并不全面——宇宙中还存在大量的其它成分。这些‘其它成分’是有关宇宙的一大未解之谜。”
据最新测算，常规物质仅占宇宙构成的4%，另有23%是一种被称为“暗物质”的神秘的隐形物质。而我们对其余高达73%的成分知之甚少，科学家们只好简称其为“暗能”。
弗拉克诺伊说：“可以想象，对那些以认识宇宙为己任的天文学家来说，每天早上起来面对镜子，却不得不说：‘我不知道大部分宇宙是由什么构成的’，他们有多么沮丧。”
旋转速率
如果暗物质不可见的话，人们何以知道它的存在呢？这一秘密的揭示源于数十年前，天文学家们注意到星系的运动十分诡异，它们旋转的速度极快——快得不可思议。
星系是由数百万甚至数十亿颗天体在旋转时因万有引力而汇聚起来的集合体。万有引力是一切物体间的相互吸引力。质量越大，引力越强。当天文学家们对星系中的物质总量进行仔细估测之后，他们发现这些物质不足以产生使旋转的星系保持聚合而不离散所必需的引力。
这一发现同样适用于我们所在的星系——银河系。“我们计算银河系中所有可见物质的质量总和，发现其总量并不足以产生维系高速运行的天体所需要的引力，”弗拉克诺伊解释道，“由此我们得出结论，银河系必定存在更多未知的引力。”
天文学家们认为，维系星系所需的额外引力来自于不可见或不可探测的物质——暗物质。然而，为了证明暗物质的存在，科学家们必须发现它们。但要发现它们谈何容易，因为无人清楚到底要找寻什么。
一些人认为，暗物质也许就是普通物质，只是它们被困在一些难以发现的地方——也许是“黑洞”或者“褐矮星”。还有一些人认为暗物质可能是一种奇异物质，它就在我们周围，但我们毫无察觉，它能穿越普通物质而无从探测。
尽管暗物质让人难以捉摸，我们仍有理由相信它的存在。它的引力大到足以使光发生弯曲，这被称为“引力透镜”现象。借助高倍望远镜，天文学家们可以看到暗物质对穿越或靠近它的光所造成的明显的扭曲变形。
宇宙膨胀
不久前，天文学家们发现了宇宙的又一奇怪现象。宇宙不仅在膨胀，而且事实上其膨胀的速度还在加快。
宇宙的膨胀可以用“大爆炸”理论来解释。该理论认为，137亿年前，宇宙由一个点向外发生爆炸。“太空在扩展，”弗拉克诺伊说，“就像你吹气球时，球体向外膨胀一样。”
由于引力的作用，宇宙的膨胀速度本应减缓，但事实上，这一过程却在加速。这种现象很反常，让你感觉如同把一只球抛向空中，它非但不落下来，反而越飞越快、越飞越高。
“如果宇宙在加速膨胀，”弗拉克诺伊说，“则肯定存在另一种形式的能量，这种能量非常强大，能够加速宇宙的膨胀。”天文学家们已经将这种形式的能量命名为暗能。但除了知道它是一种使物质分离的对抗引力的力量之外，没人有更多的了解。
弄清暗能是什么，其能量有多大，以及它对宇宙有何影响绝非小事。对这些问题的解答也许还能解决眼下一场有关宇宙最终命运的争论。宇宙会永远膨胀下去——导致“大撕裂”，还是会减缓速度，逆向发展，最终崩塌——即所谓“大坍缩”呢？
有关暗物质和暗能的谜团令弗拉克诺伊兴奋不已。“很显然，在天文学领域，我们未知的事物远比已知的多，”他说，“但如果一切问题都解决了，谁还来当科学家呢？”
1. astronomer n. 天文学家
2. weird adj. 怪诞的，不可思议的
3. fraction n. 小部分
4. invisible adj. 看不见的
5. unsolved adj. 未解决的
6. whopping adj. 巨大的
7. frustrating adj. 令人沮丧的
8. spin [spin] n. 旋转
9. tip-off 秘密消息；表明
10. galaxy n. 星系，银河
11. way [wei] adv. 非常，远远地
12. rotate v. 旋转
13. take stock of 估量，观察
14. Milky Way 银河
15. tricky adj. 棘手的，繁难的
16. telescope n. 望远镜
17. bizarre adj. 古怪的
18. ongoing adj. 进行中的
19. ultimate adj. 最终的
20. rip [rip] n. 撕裂；裂口
21. crunch n. 嘎吱的响声；紧要关头