What is the universe made of? | The Economist

The Earth, the sun, the stars, and everything we can see, only comprise five percent of the universe. But what about the other 95 percent? Scientists are puzzling over dark matter and dark energy, the mysterious components that make up the rest.

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What is the universe made of? Scientists have determined that normal matter, the stuff that makes up the earth, the stars and everything we can see, only makes up a small portion of the universe. The rest is made up of two mysterious components that are shaping our universe in profound ways. Much of the mass of the universe is made up of something called Dark Matter, which neither reflects nor emits light, but like the matter we can see, pulls things together with gravity in. Space itself seems to be permeated by an unusual force called Dark Energy which is driving things apart.

Based on current estimates scientists believe that only 5% of the universe is made up of normal matter, 27% is made up of Dark Matter, and a whopping 68% is made up of Dark Energy. So what are they and how do we know they exist?

Although we can't see Dark Matter we can tell it is out there from the effect it has on regular matter such as galaxies and stars. We can track where Dark Matter is located through an effect called gravitational lensing. According to General Relativity, massive bodies bend the fabric of space-time. That means they bend the paths of light. Astronomers can see this light bending in places where there are no visible chunks of matter, such as stars. It must be caused by Dark Matter. Through these observations, scientists have found a cosmic web of Dark Matter. Vast lumps of long threads of it. It is spread throughout the universe but tends to be concentrated in halos around galaxies. Indeed, it is considered to have been integral to the formation the large-scale structure of the universe.

So what is Dark Matter made of? No-one knows for sure yet but there are a number of theories.

There are a number of different experiments focused on finding Dark Matter, trying to catch it as it occasionally bumps into normal matter, but none of them has been successful so far. Dark Matter particles might just be created in earthly laboratories too. At the Large Hadron Collider in Switzerland, where particles smashed together near the speed of light, there is some chance that Dark Matter particles will pop out and astronomers using some of the world's largest telescopes are observing the cosmos with ever more precision to learn about where Dark Matter is located. It was through their efforts in the late 1990s that scientists learned about the other mysterious force that is shaping our cosmos - Dark Energy.

Astronomers studying distant supernovae discovered it accidentally by observing that the expansion of the universe seemed to be speeding up. Michael Turner coined the term Dark Energy to describe the mysterious force that seemed to be pushing the universe apart.

The idea that the universe could expand or contract showed up in early drafts of Albert Einstein's theory of relativity but Einstein himself believed the universe was of a fixed size. To get things to add up correctly he fudged his equations by inserting a fixed value he called a cosmological constant. Only later when it was shown the universe was in fact expanding that had become apparent that his equations were right to begin with. Einsteins cosmological constant might also end up accounting for Dark Energy. No-one knows what Dark Energy is exactly, although many theories have been postulated.

One suggestion is that it is energy folded into the fabric of space itself. As space expands, so does the amount of Dark Energy, so there will be more of it to push the universe apart. What astronomers don't know yet is if the rate of acceleration of the expansion will change over time. If it does that could have profound implications.

But the dominance of Dark Matter and Dark Energy have shifted over the lifespan of the universe. With Dark Matter playing a stronger role in the early years and Dark Energy gaining traction more recently. What will happen to our universe depends on the interplay between these two dark titans. If Dark Energy becomes more dominant the universe may thin itself out of existence in what's called the big rip. but if Dark Matters influence should increase, that could collapse the universe back upon itself in a Big Crunch. And if neither force changes dramatically space may just continue expanding outward indefinitely.

For cosmologists trying to foresee the ultimate fate of the universe much remains in the dark.

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