Voyage to the Heart of Dark Matter

Dark matter neither emits nor absorbs light. But its gravitational influence on galaxies betrays its presence.
Credit: Amercian Museum of Natural History
Matière sombre
  • Matière sombre
  • La galaxie d’Andromède
  • Vera Rubin, au télescope de 84 pouces à Kitt Peak, 1970.
Voyage to the Heart of Dark Matter

When we look at the night sky, the luminous points of light we see are stars. But is the space between the stars empty? For a long time that’s what we believed. In reality, astronomers have discovered that the luminous part of the universe we can see amounts to just 5%. The bulk of the cosmos consists of “dark matter” and “dark energy.”

Dark matter is discovered

From Earth, astronomers have discovered many amazing things about the universe. On a clear dark night we can see a luminous band stretching across the sky: It’s our galaxy, the Milky Way, viewed edge-on from within. Beyond our Milky Way there are over a hundred billion other galaxies. The closest spiral galaxy to us is the famous Andromeda galaxy situated about 2.5 million light-years away.

Back in the 1970’s, American astronomer Vera Rubin carefully measured the rotation of the Andromeda galaxy. But based on the visible mass of its stars and other material, the galaxy was turning too quickly for its gravity to keep it together: it should be spinning apart. The mostly likely solution: a large amount of unseen mass must be keeping the galaxy intact. But the Andromeda galaxy is not alone: calculations now show that galaxies are held together by at least ten times more mass than we can see. All the visible material we see represents a small part of a universe that’s a lot more mysterious and complex that we ever imagined…a universe laced with dark matter!

But what about dark energy

We can’t see dark matter because it doesn’t emit or absorb light. But its gravity interacts with ordinary matter, such as stars and galaxies, which we can see. The gravity generated by dark matter is necessary for the formation of the large-scale filamentary structure that we observe in the universe today. Yet despite all the mystery surrounding dark matter, we’ve recently discovered an even greater cosmic enigma.

Astronomers predicted that the gravity generated by dark matter would slow down the expansion of the universe. Yet in 1998 a group of researchers measured the velocity of distant supernova explosions and discovered that the expansion of the universe was accelerating! The internal pressure causing this acceleration is called dark energy.

But the nature of this dark energy is a complete enigma. What we do know is that it represents 70% of the mass-energy content of the universe; another 25% is dark matter, and the remaining 5% is ordinary matter—the stuff we’re made of.

It turns out that the universe is a lot more complex and mysterious than we ever imagined, and for every question we answer, two more questions arise.

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