Dark Matter
Dark matter is one of the greatest mysteries of modern physics. It neither emits nor absorbs light, making it completely invisible to our telescopes. Yet astronomers are convinced of its existence because of its gravitational effects on visible matter.
Why do we call it dark matter?
In the 1970s, astronomer Vera Rubin observed something strange: stars on the outskirts of galaxies were orbiting just as fast as those at the center. However, according to Newtonian mechanics, they should be orbiting much more slowly, like the distant planets in the solar system. The only coherent explanation is the presence of an enormous amount of invisible matter that amplifies gravity—dark matter.
What We Know (and What We Don’t Know)
Dark matter is thought to account for about 27% of the universe’s content, compared to only 5% for ordinary matter (stars, planets, gas). It does not appear to interact with light or ordinary matter, except through gravity. It forms invisible structures called halos that surround galaxies and hold them together.
However, we still do not know what it is made of. The most serious candidates include hypothetical particles such as WIMPs (weakly interacting massive particles) or axions. Despite decades of research, no direct detection has yet been confirmed.
Why is it important?
Without dark matter, galaxies as we observe them could not exist: they would fall apart under the effect of their own rotation. Dark matter is also the invisible framework upon which the structure of the universe was built after the Big Bang— forming the filaments and galaxy clusters that we observe on a large scale.
A concrete example: gravitational lensing
One of the most striking pieces of evidence for dark matter comes from observations of collisions between galaxy clusters. In the case of the Bullet Cluster, two clusters collided. Ordinary matter was slowed down and distorted by the impact, but dark matter, whichinteracting with itself, passed right through without slowing down. Gravity maps obtained through gravitational lensing clearly show that the mass is separated from the visible matter.
Did you know?
If we gathered all the dark matter in the universe and made it visible, theuniverse would look like a vast cosmic spiderweb, with filaments connecting galaxy clusters to one another.
Frequently Asked Questions
Can we see dark matter?
No. Dark matter does not emit or absorb light. It can only be detected indirectly, through its gravitational effects on visible matter or on the path of light.
Is dark matter dangerous?
No. Dark matter constantly passes through us without any detectable effect. It interacts so weakly with ordinary matter that its presence is completely harmless.
What is the difference between dark matter and dark energy?
Dark matter is a form of invisible matter that acts through gravity and holds galaxies together. Dark energy, on the other hand, is a mysterious force that causes the universe to expand at an accelerating rate. These are two distinct phenomena, although both remain unexplained.