Concept preview. Not indexed. Facts are checked against the sources listed on each page.
Dark matter: the biggest thing we cannot see
Most of the matter in the universe gives off no light. We know it is there from what it does to everything else.
In human terms
- Of all the energy and matter in the universe, ordinary stuff (stars, planets, you) is about 5%. Dark matter is about 27%. The remaining 68% is dark energy.
- Dark matter is thought to outweigh the ordinary matter in a galaxy by about 5 to 1.
Measured: we know this
- Stars at the edge of galaxies orbit too fast for the visible matter alone to hold them in.
- Gravitational lensing maps show mass where there is no visible matter.
- The pattern of the cosmic microwave background fits a universe with about five times more dark matter than ordinary matter.
Inferred: we think this
- Dark matter is a kind of particle that barely interacts with light or normal matter.
- It is "cold", meaning slow moving, which helps galaxies form.
Unknown: we do not know
- What the particle is, or whether it is a particle at all.
- Underground detectors have run for decades and have not yet confirmed a detection.
- Whether modified gravity theories can replace it. Most evidence disfavours that, but the question is not closed.
What would settle it
- A confirmed signal in a direct-detection experiment.
- Making the particle in a collider and matching it to the sky.
- An observation that dark matter alone cannot explain and modified gravity can.
Sources
Planck 2018 cosmological parameters (ESA). NASA Science: Dark Energy, Dark Matter. Concept preview: verify the percentages against the cited sources before launch.