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Looking up at the night sky, we are amazed by how it seems to go on forever.
仰望夜空，我們為它的永無止境而感到驚訝。
But what will the sky look like billions of years from now?
然而數十億年前的天空又會是什么樣子呢？
A particular type of scientist, called a cosmologist, spends her time thinking about that very question.
有一類科學家被稱為宇宙學家，他們就專門思考這樣的問題。
The end of the universe is intimately linked to what the universe contains.
宇宙的盡頭與其自身的構成有著密切的聯系。
Over 100 years ago, Einstein developed the Theory of General Relativity, formed of equations that help us understand the relationship between what a universe is made of and its shape.
一百多年前，愛因斯坦提出了廣義相對論，以方程的形式幫助我們理解宇宙的組成與其形狀之間的關系。
It turns out that the universe could be curved like a ball or sphere.
結果表明，宇宙可能是呈球狀或半球狀的曲線。
We call this positively curved or closed.
以方程的形式
Or it could be shaped like a saddle.
或者它也可能呈馬鞍狀。
We call this negatively curved or open. Or it could be flat.
我們稱之為負彎曲或負開放。又或者它也可能是扁平狀。
And that shape determines how the universe will live and die.
形狀決定著宇宙將會怎樣存在和滅亡。
We now know that the universe is very close to flat.
如今，我們認為宇宙最接近扁平狀。
However, the components of the universe can still affect its eventual fate.
然而，宇宙的其他構成也會影響其最終的命運。
We can predict how the universe will change with time if we measure the amounts or energy densities of the various components in the universe today.
我們能夠預測宇宙如何隨時間發生改變，這需要我們測量目前組成宇宙的各要素數量及其能量密度。
So, what is the universe made of?
那么，宇宙是由什么構成的呢？
The universe contains all the things that we can see, like stars, gas, and planets.
宇宙中包含我們肉眼所能看到的一切，比如恒星、氣體和行星。
We call these things ordinary or baryonic matter.
我們稱這些為普通物質或重子物質。
Even though we see them all around us, the total energy density of these components is actually very small, around 5% of the total energy of the universe.
盡管它們就在我們周圍，但這些組成部分的總能量密度卻十分渺小，它們僅占整個宇宙總能量的5%左右。
So, now let's talk about what the other 95% is.
那么現在，我們就來探討組成宇宙另外95%的部分是什么。
Just under 27% of the rest of the energy density of the universe is made up of what we call dark matter.
在剩余的宇宙能量密度中，近27%是由暗物質構成的。
Dark matter is only very weakly interacting with light, which means it doesn't shine or reflect light in the way that stars and planets do, but, in every other way, it behaves like ordinary matter—it attracts things gravitationally.
暗物質與光的相互作用很弱，也就是說暗物質本身并不發射或反射光線，這與其他恒星與行星不同，但從其它方面來講，暗物質與普通物質相似——暗物質通過引力吸引物體。
In fact, the only way we can detect this dark matter is through this gravitational interaction, how things orbit around it and how it bends light as it curves the space around it.
事實上，我們唯一能夠探測暗物質的方法就是通過它與其它物質的引力相互作用，比如物體如何圍繞暗物質運行或暗物質如何扭曲光線，使其周圍空間呈現曲線狀。
We have yet to discover a dark matter particle, but scientists all over the world are searching for this elusive particle or particles and the effects of dark matter on the universe.
目前我們還沒能發現一塊暗物質顆粒，但全世界的科學家們都在探索這個或這些神秘物質，以及它對宇宙產生的影響。
But this still doesn't add up to 100%.
可是這樣的組成還是未能達到100%。
The remaining 68% of the energy density of the universe is made up of dark energy, which is even more mysterious than dark matter.
剩余68%的宇宙能量密度來自于暗能量，它比暗物質更加神秘。
