But what will the sky look like billions of years from now? 然而数十亿年前的天空又会是什么样子呢？

　　A particular type of scientist, called a cosmologist, spends her time thinking about that very question. 有一类科学家被称为宇宙学家，他们就专门思考这样的问题。

　　The end of the universe is intimately linked to what the universe contains. 宇宙的尽头与其自身的构成有着密切的联系。

　　Over 100 years ago, Einstein developed the Theory of General Relativity 一百多年前，爱因斯坦提出了广义相对论，

　　formed of equations that help us understand the relationship between what a universe is made of and its shape. 以方程的形式帮助我们理解宇宙的组成与其形状之间的关系。

　　It turns out that the universe could be curved like a ball or sphere. 结果表明，宇宙可能是呈球状或半球状的曲线。

　　We call this positively curved or closed. 以方程的形式

　　Or it could be shaped like a saddle. 或者它也可能呈马鞍状。

　　We call this negatively curved or open. Or it could be flat. 我们称之为负弯曲或负开放。又或者它也可能是扁平状。

　　And that shape determines how the universe will live and die. 形状决定着宇宙将会怎样存在和灭亡。

　　We now know that the universe is very close to flat. 如今，我们认为宇宙最接近扁平状。

　　However, the components of the universe can still affect its eventual fate. 然而，宇宙的其他构成也会影响其最终的命运。

　　We can predict how the universe will change with time 我们能够预测宇宙如何随时间发生改变，

　　if we measure the amounts or energy densities of the various components in the universe today. 这需要我们测量目前组成宇宙的各要素数量及其能量密度。

　　So, what is the universe made of? 那么，宇宙是由什么构成的呢？

　　The universe contains all the things that we can see, like stars, gas, and planets. 宇宙中包含我们肉眼所能看到的一切，比如恒星、气体和行星。

　　We call these things ordinary or baryonic matter. 我们称这些为普通物质或重子物质。

　　Even though we see them all around us, 我们能够预测宇宙如何随时间发生改变，

　　the total energy density of these components is actually very small, 但这些组成部分的总能量密度却十分渺小，

　　around 5% of the total energy of the universe. 它们仅占整个宇宙总能量的5%左右。

　　So, now let's talk about what the other 95% is. 那么现在，我们就来探讨组成宇宙另外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%是由暗物质构成的。

　　Dark matter is only very weakly interacting with light, 暗物质与光的相互作用很弱，

　　which means it doesn't shine or reflect light in the way that stars and planets do, 也就是说暗物质本身并不发射或反射光线，这与其他恒星与行星不同，

　　but, in every other way, it behaves like ordinary matter—it attracts things gravitationally. 但从其它方面来讲，暗物质与普通物质相似——暗物质通过引力吸引物体。

　　In fact, the only way we can detect this dark matter is through this gravitational interaction, 事实上，我们唯一能够探测暗物质的方法就是通过它与其它物质的引力相互作用，

　　how things orbit around it and how it bends light as it curves the space around it. 比如物体如何围绕暗物质运行或暗物质如何扭曲光线，使其周围空间呈现曲线状。

　　We have yet to discover a dark matter particle, 目前我们还没能发现一块暗物质颗粒，

　　but scientists all over the world are searching for this elusive particle or particles 但全世界的科学家们都在探索这个或这些神秘物质，

　　and the effects of dark matter on the universe. 以及它对宇宙产生的影响。

　　But this still doesn't add up to 100%. 可是这样的组成还是未能达到100%。

　　The remaining 68% of the energy density of the universe is made up of dark energy, 剩余68%的宇宙能量密度来自于暗能量，

　　which is even more mysterious than dark matter. 它比暗物质更加神秘。