What Is Dark Matter? The Invisible 27% of the Universe
Dark matter is one of the greatest mysteries in science. It is an invisible substance that makes up about 27% of the universe — roughly five times more than all the ordinary matter in stars, planets and people combined. We have never directly seen it, yet the evidence that it exists is overwhelming.
How we know it’s there
We can’t see dark matter, but we can see its gravity pulling on things we can observe:
- Galaxies spin too fast. The stars at the edges of galaxies orbit so quickly that, based on visible matter alone, they should fly apart. Something invisible must be adding gravity to hold them together.
- Gravitational lensing. Massive objects bend the light of galaxies behind them. The bending is far stronger than visible matter can account for — revealing extra, unseen mass.
- The cosmic microwave background. The pattern of the Big Bang’s afterglow only matches our observations if dark matter was present in the early universe.
What could it be?
That’s the open question. Dark matter doesn’t emit, absorb or reflect light, and it barely interacts with ordinary matter except through gravity. Leading candidates:
- WIMPs (Weakly Interacting Massive Particles) — hypothetical heavy particles, long the favourite, though direct searches haven’t found them yet.
- Axions — extremely light hypothetical particles, now a major focus.
- Primordial black holes — a minority idea.
Experiments deep underground and at particle colliders are hunting for it, so far without a confirmed detection.
What it is not
Dark matter is not the same as dark energy. They’re often confused, but they do opposite things:
- Dark matter adds gravity — it pulls things together and builds cosmic structure.
- Dark energy pushes the universe apart, accelerating its expansion.
The cosmic recipe
- 68% dark energy
- 27% dark matter
- 5% ordinary matter
Everything humanity has ever seen — every star and galaxy — is that last 5%. The rest of the universe remains, quite literally, in the dark.
Related: what is antimatter?
FAQ
What is dark matter in simple terms?
An invisible form of matter that doesn't emit light but exerts gravity, holding galaxies together and making up about 27% of the universe.
How do we know dark matter exists if we can't see it?
Through its gravity — galaxies spin too fast, light bends too much, and the early universe's structure all require extra unseen mass.
Is dark matter the same as dark energy?
No. Dark matter pulls things together with gravity; dark energy pushes the universe apart. They are distinct mysteries.