We don't know if dark matter exists. So why do astronomers keep looking?

Spiral galaxies like M100, pictured here, may hold answers about the nature of dark matter. NASA Spitzer Space Telescope/NASA/JPL-Caltech

Scientists know very little about the matter that makes up the galaxies in the Universe. About 20% of the matter in galaxies is visible or baryonic: subatomic particles like protons, neutrons and electrons. The other 80%, referred to as “dark matter”, remains mysterious and unseen.

In fact, it may not exist at all. “Dark matter” is just a hypothesis. Physicists and astronomers may be chasing a phantom – but that doesn’t stop us from looking. Why? Because if dark matter isn’t real, then the behaviour of the stars, planets and galaxies makes little sense.