Scientists have been puzzling over dark energy since the 1990s, and they still don’t know exactly what it is. But they have an idea: that there’s some kind of strange energy making up roughly 70% of the universe, which is pushing galaxies away from each other at an accelerated rate and causing the universe to expand at an increasing rate overall.
How did we figure out there was such a thing as dark energy?
The cosmic microwave background is kind of like a baby picture of our entire universe. It shows us how everything looked right after it was born, some 13.8 billion years ago. Since we can’t see anything earlier than that, cosmologists want to use other methods to study conditions earlier in time. The best way they’ve found to do that is by looking at distant supernovae and other exploding stars – which should be dimmer than normal since everything was closer together back then. Instead, they’re brighter. That tells astronomers there must have been more matter (and thus more gravity) holding those objects together early on, before things spread out as the universe aged. In short, dark energy seems to make up about 70 percent of all matter and energy in our cosmos. Scientists are still trying to figure out what exactly dark energy is – but so far it looks like Einstein might have been right again when he first predicted its existence over 100 years ago.
Where does this go from here?
Most of the matter we can see only makes up 5% of our universe, while dark matter accounts for 25%. The remaining 70% is made up of dark energy, which scientists are still trying to understand. So far, they know that it causes galaxies to expand at an accelerated rate and keeps everything moving away from each other—and has been doing so since time began. In fact, astronomers have calculated that if dark energy didn’t exist, then gravity would pull all matter together into one huge cosmic clump instead of allowing us to live in a vast expanse with billions of galaxies and trillions upon trillions of stars. It’s hard not to be impressed by something like that.
There’s also an excellent PBS video called What Is Dark Matter? that discusses dark energy as well as another type of matter called dark matter. That said, don’t let your curiosity get you down; despite how much scientists know about dark energy today, there are still many things about it that remain unknown.
Why does it matter what the universe is made of?
A great deal of effort has gone into determining what makes up our universe. Our lives are affected by how much dark energy there is, for example, as it drives galaxies away from each other—and if a galaxy is moving too fast, its gravity will not be strong enough to prevent stars escaping its grasp forever. Similarly, we need to know what makes up a black hole in order to understand why they exist and whether or not they can be used as tools for interstellar travel. So far, astronomers have determined only that dark energy accounts for 68% of all matter in existence and 27% of all mass-energy (the stuff which causes gravity). There’s still plenty more work to do!
Luckily, scientists don’t just have their eyes on measuring what exists. They want to know how things came to be; and thanks to Einstein’s Theory of General Relativity, we now at least partly understand where those questions may take us. For example, dark matter is thought to exist because galaxies are spinning too quickly for it to all be accounted for by visible matter (such as stars) alone. If a galaxy were made up only of visible matter (like our own), it would spin much more slowly than it does—so there must be some invisible mass out there causing it to spin faster than expected.
