One issue with measuring the mass of galaxies is the galaxy rotation curve problem. When an ice skater pulls in their arms during a spin, you would expect them to move faster as their mass is more concentrated towards their centre. So in a galaxy, you’d also expect the stars nearer the centre to move faster, but that’s not the case. Instead we find that stars towards the edge of a rotating galaxy actually move faster, not slower. Scientists think the answer is the presence of invisible dark matter. The gravitational tug of dark matter causes the stars at the edge to move faster than theory predicts and thereby helps iron out any discrepancies in the measurement of a galaxy’s gravity and mass.
Dark matter is thought to affect the rotation rate of galaxies