Why do scientists think dark matter exists?

Astronomers notice that stars in galaxies orbit faster than expected from the visible stars, gas, and dust alone. Galaxy clusters also show gravitational effects that imply much more mass than what telescopes can see. Gravitational lensing, where gravity bends light, provides another strong clue that unseen matter is present. Dark matter does not emit, absorb, or reflect light, which is why it remains invisible to ordinary telescopes. The evidence has accumulated since the work of Fritz Zwicky in the 1930s and Vera Rubin’s galaxy rotation studies in the 1970s.