Should we be so sure that ‘dark matter’ exists?

It has been more than 50 years since astronomers first proposed “dark matter”, which is thought to be the most common form of matter in the universe. Despite this, we have no idea what it is – nobody has directly seen it or produced it in the lab.

So how can scientists be so sure it exists? Should they be? It turns out philosophy can help us answer these questions.

Back in the 1970s, a seminal study by astronomers Vera Rubin and Kent Ford of how our neighbour galaxy Andromeda rotates revealed a surprising inconsistency between theory and observation. According to our best gravitational theory for these scales – Newton’s laws – stars and gas in a galaxy should rotate slower and slower the further away they are from the galaxy’s centre. That’s because most of the stars will be near the centre, creating a strong gravitational force there.