Dark matter and dark energy

Everything we see (including us) consists of particles, from the largest star to the smallest atom, and even the even smaller building blocks that atoms are made of.  Particles have  weight and take up space, meaning they have mass and volume. Particles also interact with light, which is what allows us to see things. If you look at a flower, what you are actually seeing is light from the Sun or a lamp hitting the flower's particles and being reflected into your eyes. We call all these particles matter, which is what gives us mass and volume, and what interacts with light.

In addition to this matter, the universe also consists of two other things: dark matter and dark energy. Today we believe that all "regular" matter makes up only around 5% of the universe. This means that everything we can see around us in the universe makes up only a very small part of the whole! Since this is the case, what do the remaining 95% of the universe consist of? Researchers believe that around 27% of the universe is dark matter, and that the remaining 68% is dark energy. 
 

Dark matter

The term dark matter is a little misleading as it is not actually dark, but rather invisible. Dark matter does not interact with light in any way. Unlike black holes, which are also invisible but actively absorb and trap light, dark matter simply ignores it entirely. It neither absorbs, reflects, nor emits light, as if light does not even know it is there. You can read more about black holes and how they interact with light in the section on Black Holes. This complete invisibility makes dark matter extraordinarily difficult to observe, because almost everything we know about the universe comes from the light objects emit or reflect. This is why we do not have a lot of information on what dark matter is. What we do know is that it is there because we can measure its gravitational effect, which is precisely the way that dark matter was discovered in the first place in the early 20th century.

At that time, a number of scientists looked at the movement of stars around the centre of the Milky Way, and later at how other galaxies rotate. It turned out that the stars were not moving as they should around the galaxy. They were moving so fast that the gravity from all the dust, gas, stars, and planets should not have been enough to hold them together. It could therefore be calculated that there must be something else in the galaxies helping to hold them together, something we cannot observe but which has mass, and that is what we call dark matter. Today researchers have found several ways to measure dark matter even though it remains invisible to us.

In the image below we see a collision of two galaxy clusters. The pink area marks hot gas, ordinary matter, from the two clusters interacting, while the blue areas mark where the gravity of dark matter is bending space. Because the ordinary and dark matter lie in different areas during the collision, the dark matter can be clearly identified. Researchers are working intensively to find out what dark matter could be, but so far there is no consensus.

One suggestion is that dark matter could be what we call weakly interacting massive particles (WIMPs). These hypothetical particles are large, heavy, and slow, and they do not interact with light or very much with other particles. Researchers are trying to detect these particles using various detectors or create them in laboratories, but so far without success.

Another suggestion is axions, hypothetical elementary particles that are very light, have low energy, and interact only very weakly with other particles. These particles have also been proposed as the solution to a problem in quantum chromodynamics, the branch of physics that describes how quarks and the forces between them behave, the same quarks we mentioned back in The Big Bang section. Therefore, if researchers one day prove that axions exist, we could shed some light on two major current unsolved mysteries!

The final suggestion is that dark matter is in fact so-called primordial black holes. Primordial black holes are hypothetical black holes that researchers believe may have formed during the birth of the universe. At that time the universe was unimaginably dense, and researchers think black holes may have arisen spontaneously. These black holes could be as small as atoms and as large as supermassive black holes, and they could now be scattered throughout the universe, where we feel their gravity but cannot see their light. Read more Black Holes and The Big Bang if you want to know more about these topics!

Dark energy

Dark energy and dark matter are two very different things. The only thing they have in common is that they do not interact with light and therefore cannot be seen directly, which has given them the name "dark."

In 1929, astronomer Edwin Hubble looked closely at some distant galaxies and found something very strange. He could see that the galaxies were all moving away from us, and the further away they were, the faster they were moving. Today we know that it is not actually the galaxies that are moving, but rather the universe that is expanding, and therefore the space between galaxies is getting larger. This discovery puzzled researchers, since we know that gravity should pull things closer together, not push them further apart! In the 1990s there was a widespread theory that the expansion of the universe probably originated with the Big Bang, but that gravity would at some point slow this expansion and eventually cause the universe to contract and collapse. This theory is called the Big Crunch.

In 1990, the Hubble Space Telescope was launched, and with it we could see further out into the universe than ever before, as when we look out into the universe, we are also looking back in time. In fact, we are observing light from very distant objects and galaxies, and the light we see has taken time to reach us on Earth. The further away the galaxies are, the more time the light has taken. It is a bit like receiving a letter: if you receive a letter from your neighbour, they probably just wrote it, but if you receive a letter from the other side of the world, it is probably a week old when it arrives, and it will tell you about things that happened a week ago!

When the Hubble Space Telescope looks at distant galaxies, it sees light that has taken billions of years to reach us, meaning it observes those galaxies as they looked billions of years ago. By comparing how fast galaxies were moving apart back then versus today, astronomers made a shocking discovery: the expansion of the universe is not slowing down as expected, it is actually speeding up. This was the complete opposite of what scientists had believed in the 1990s, when the prevailing theory was that gravity would gradually put the brakes on the expansion and eventually pull everything back together.

To this day researchers are not entirely sure what is causing the universe to expand faster and faster when gravity should be pulling it together. It must be a force that works against gravity, and there must be a great deal of it. This force has been given the name dark energy. As dark energy cannot be observed directly, very little is known about it, and we can only look at how it affects the visible universe. A few scientists believe that dark energy is a property of the vacuum. That empty space has its own energy that causes it to expand, and the more empty space there is between galaxies, the more it will expand.

This idea of vacuum energy was actually anticipated by Albert Einstein. When he first developed his general theory of relativity, his equations kept suggesting that the universe should either be expanding or contracting, but at the time most scientists believed the universe was static and unchanging. To make his equations produce a stable, non-expanding universe, Einstein introduced what he called the cosmological constant, essentially an extra term that counteracted gravity and kept everything in balance. When Hubble's observations later showed that the universe was in fact expanding, Einstein realised his cosmological constant had been unnecessary, and reportedly called it his greatest mistake. But here is the twist: now that we know the expansion is not just happening but actually accelerating, something very much like Einstein's cosmological constant seems to be needed after all to explain it. It turns out Einstein may have had the right idea for the wrong reasons, one of the most remarkable reversals in the history of science!

The future of the universe

While dark matter has gravity and helps hold things together, dark energy works against gravity and pushes things apart. The balance between dark matter and dark energy is therefore crucial for the future development of the universe. We know so little about both that it is very difficult to say anything precise about what will happen to our universe in the future. Nevertheless, researchers have put forward a few possibilities for how the universe's fate might unfold.

The Big Crunch 

If there is not enough dark energy in the universe, the gravity from dark matter and ordinary matter could halt the expansion and eventually pull the universe back together in a kind of reverse Big Bang. Some believe that when the universe has collapsed to a tiny point, it could explode again in a new Big Bang, creating a new universe. Perhaps we are just one in a long series of universes.

The Big Chill 

If dark energy is stronger than dark matter, our universe will continue to expand. Over time, all galaxies will lie so far from each other that we will eventually no longer be able to see the light from other galaxies. Galaxies will continue to form stars for around another trillion years, but after that there will be no more gas left and no new stars will form. The universe will grow dark as the stars go out one by one.

Other fates

Since we do not know what dark matter or dark energy are, it is impossible to know what will happen to the universe in the future. There are therefore many different possibilities. Perhaps our universe will disappear just as suddenly as it appeared? Or perhaps there are many more universes than just our own, and we are just a tiny speck in an infinite multiverse? One thing is certain: we will never stop asking questions. And perhaps that insatiable curiosity is the most human thing of all.