Our universe is big. Like really, really big, and it is growing larger every second. However, we actually don’t understand or know what most of the universe is made of. The known universe, Earth, the Sun, other planets, and stars are all composed of atoms. This “ordinary” or baryonic matter only makes up less than 5 percent of the universe. The rest of the universe is made up of this stuff that we can’t see and don’t really understand, known as dark matter and dark energy.


If we can’t see it, how do we even know it’s there in the first place?

Scientists have never directly seen dark matter. It is invisible to everything because it does not interact with baryonic matter and is invisible to light and radiation, so none of the modern instruments we have can see it. However, we can see the effects that it has on the universe.  Dark matter and dark energy have gravitational effects on galaxies and galaxy clusters. (A galaxy cluster is hundreds of galaxies bound together.) Dark matter and energy can distort and magnify light from nearby galaxies and can also affect the gravity of stars. We know that these strange occurrences are dark matter and dark energy because nothing else measurable in the universe can explain it. 

What is dark matter?

Dark matter makes up about 27% of the universe. We don’t really know what it is, but we do know what it is not. First, and most obviously, it is dark so it is not in the form of the planets and stars that we can see.  Second, it is not in the form of a  cloud of normal matter, the matter that is made of baryons. We know this because baryons absorb radiation and we would be able to sense that. We know that dark matter is not antimatter because if it was, we would see special gamma rays that are created when antimatter obliterates matter. (Antimatter is basically the arch-nemesis of matter and they do not get along. If they touch they both explode and create pure energy.)We also can rule out really big black holes (like galaxy-sized) because of the number of gravitational lenses that we see.

We do have two ideas about what dark matter could be.  Dark matter could be still made up of baryonic matter if it is tied with brown dwarfs (brown dwarfs are objects with a mass that is between the heaviest gas planets and the lightest stars) in small dense chunks of heavy elements. That possibility is known as massive compact halo objects, or “MACHOs”. The more likely possibility is that dark matter is not baryonic at all and that it is made up of more exotic particles like WIMPS (Weakly Interacting Massive Particles). (Scientists are great at naming things.)


What is dark energy?

Dark energy is even more mysterious than dark matter, even though it makes up about 68% of the universe. When scientists first discovered it in the 1990s, they were shocked. Before this discovery, physicists thought that over time, the expansion of the universe would slow down because of gravity. However, when they ran tests, they realized that the universe is actually growing faster than it was before. Scientists now think that the fast expansion of space is because of repulsive force generated by quantum fluctuations (temporary random change in the amount of energy in a point in space) in otherwise “empty” space. Scientists called this energy dark energy. (Because once again, scientists are really bad at naming things.)

Scientists really do not have a plausible explanation for dark energy yet. One theory is that dark energy is the fifth type of fundamental force, called quintessence, which fills the universe like a fluid. Scientists also point out that the known properties of dark matter match up with a cosmological constant, a mathematical Band-Aid of sorts that Albert Einstein added later to his theory of relativity to fit with the idea of a still universe. According to Einstein, the constant is a repulsive force that counteracts gravity, basically keeping the universe from collapsing on itself. (Which would not be good, to say the least.) However, Einstein later discarded the idea when it was revealed that the universe was not static. Now we know that the universe is accelerating in its expansion and we can add in dark energy as a cosmological constant, which would neatly explain how space-time is being stretched. However, this still does not explain why this force exists in the first place.

What is the difference?

Dark matter and dark energy are basically playing tug-of-war with the universe. Dark matter is trying to slow down the expansion of the universe while dark energy is trying to speed it up. Dark matter is like cement, pulling everything together because it interacts with gravity, but not with light. Dark energy is a repulsive force that is driving the expansion of the universe.


Why are they so important?

Dark matter and dark energy are super important because they make up about 95% percent of the universe. That’s a lot of matter and energy that we don’t really understand. If we can figure out what they are for and how they are created it could explain a lot of the universe. That’s a pretty big deal!


Now you’re not in the dark about dark matter and dark energy anymore.


Sources:

https://www.nationalgeographic.com/science/article/dark-matter
https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy

https://astronomy.com/news/2020/03/whats-the-difference-between-dark-matter-and-dark-energy