Everything we can see — stars, planets, gas, dust, people — makes up only a small part of the universe. Measurements suggest that ordinary matter accounts for about 5% of its total energy content. Dark matter makes up roughly 27%, and the rest is an even more mysterious dark energy.

The clues

Galaxies spin too fast

In the 1970s, astronomers including Vera Rubin measured how fast stars orbit in spiral galaxies. Stars far from the centre moved just as fast as those closer in — too fast to be held by the visible matter alone. Something unseen was adding gravity.

Light bends around invisible mass

Massive objects bend light passing near them, an effect called gravitational lensing. Lensing around galaxy clusters reveals far more mass than their stars and gas contain.

What could it be?

Dark matter does not appear to be made of atoms. Leading candidates include:

  • WIMPs — hypothetical weakly interacting massive particles.
  • Axions — very light particles originally proposed to solve a different puzzle in physics.
  • Other ideas, including modifications to the laws of gravity instead of new particles.