[Astrophysics] Is there something invisible in the universe? That's dark matter.

──Something that is invisible yet shapes the universe

Dark matter. Also known as dark matter.
It is a sci-fi term that is just as impressive as the dark energy I wrote about in a previous article (laughs). However, its existence is also scientifically predicted.
Although it has a name that sounds like you might fall into darkness if you touch it, this dark matter is actually considered to be a fairly common substance in the universe.

1. Is there "something invisible" in the universe?

When you look at the stars and galaxies in the night sky, the universe seems to be filled with light. However, according to astronomical research, the matter we can see with our eyes (stars, planets, gas, dust, etc.) accounts for only about 5% of the entire universe. About 27% of the remainder is thought to be a mysterious substance called "dark matter," which does not emit light and cannot be directly observed by our eyes.
Why is it thought to exist even though it doesn't shine and cannot be observed?

2. How do we know dark matter exists?

You might wonder, "How did you find out about something invisible?" The key was the observation of galactic rotation.

The stars in a galaxy are bound to the center by gravity, and just as planets orbit the sun, the stars in a galaxy revolve around the center of the galaxy.
The rotation speed of a star revolving around a gravitational source is determined by the strength of the gravity and the distance from the center. For celestial bodies revolving around the same gravitational source, the rotation speed decreases as the distance from the center increases. This is called Kepler's third law.
Since a galaxy is made up of many stars, it cannot be calculated as simply as planets orbiting the sun, but even so, the rotation speed of each celestial body can be calculated to some extent using this law. This is because the strength of gravity at each position within the galaxy can be calculated to some extent.

However, the actual observation results were unexpected.
Both the outer stars and the inner stars were rotating at almost the same speed.

This clearly contradicts Kepler's third law. This problem is called the "galaxy rotation curve problem," and it had become a fairly definitive problem by the 1970s.
Even though the rotation speed should decrease with distance from the center for the same gravitational source, it was not decreasing (in fact, there were observations where the outer stars were rotating faster!).
Since the law determines the rotation speed based on the strength of the gravitational source and the distance, if the law is correct, gravity must be stronger for the rotation speed to be faster.
Gravity should have been calculated from the calculations, but the observation results cannot be explained unless gravity is even stronger.
The inference drawn from this is, is "something invisible" adding gravity...?

In addition, various observations such as the movement of galaxies when they collide, the measurement of the total mass of galaxy clusters, and the gravitational lensing effect (a phenomenon where light bends significantly) support the existence of "a large amount of something invisible."

If we consider that the gravity caused by the mass of this invisible something is having an effect, various problems can be explained well. So, we decided to call this invisible something dark matter.

3. Why dark matter doesn't shine

Although the existence of this dark matter is certainly predicted, it cannot be seen, and no data indicating its existence appears in any observation results.
In astrophysics, this inability to be directly observed is sometimes described as "dark."
Dark matter is thought to be dark because it hardly emits or absorbs electromagnetic waves. Ordinary matter interacts with light and electromagnetic waves, but dark matter hardly does so, so it does not appear in our telescopes. Gravity is the only clue to its existence. It is like an image of "shadow dwellers" supporting the framework of the universe.

4. Candidates for its identity

The identity of dark matter has not yet been determined. There are several candidates, but all are still in the verification stage, and no evidence has been found.

・WIMPs (Weakly Interacting Massive Particles): Particles that are theoretically heavy and stable, and interact with almost nothing except gravity. Searches are underway using special underground detectors.

・Axions: Hypothetical, extremely light particles. They have been proposed as an existence that complements theories of the early universe.

・Groups of massive celestial bodies: It was thought that black holes or dark dwarf stars might collectively account for the mass, but observational results show that this alone is not enough.

5. Building the framework of the universe

Dark matter is not just an "invisible lump." Rather, it is thought to play a role like a "blueprint" that builds the structure of the universe.

It was thought that a long time was needed to form galaxies, but observations have revealed that mature galaxies existed even in the early universe. This cannot be explained by the matter we can currently observe.
However, if we take the existence of dark matter into account, it can be explained well. It is thought that because dark matter first created a gravitational "scaffold," ordinary matter gathered, and galaxies and galaxy clusters were formed. Our galaxy and Earth were also born on top of that invisible framework.

6. Mysteries hidden in everyday life

It is a strange feeling to think that although dark matter cannot be felt directly, it supports our existence from the shadows. For example, the arrangement of stars in the night sky and the beautiful spiral of the Milky Way galaxy are maintained because the gravity of dark matter is in the background.
When you think about this "other material of the universe" that is usually invisible, doesn't the familiar starry sky feel more mysterious?

7. Future Exploration

The challenge to identify dark matter continues. Research is being conducted from various angles, including large-scale detectors on the ground, space telescopes, and accelerator experiments. If its true nature is revealed, it will open the door not only to understanding the evolution of the universe but also to new laws of physics.

Furthermore, if we can more accurately understand how dark matter shapes the gravitational map of the entire universe, we should be able to draw a more precise scenario of the universe's growth from the Big Bang to the present day.

◇ Summary

Dark matter is a mysterious substance that accounts for about a quarter of the universe, yet cannot be seen with light. Its existence is considered almost certain due to phenomena such as galaxy rotation and gravitational lensing, but its true nature remains a mystery.

Even so, without dark matter, the current structure of the universe and perhaps even our own existence might not have been possible. Something that is invisible but definitely there—knowing this is a major step toward understanding the origins of the universe.