How do astronomers detect dark matter if they cannot see it?

Dark matter does not emit, absorb, or reflect light in the ways ordinary matter does, so astronomers search for its gravitational effects. Galaxy rotation curves stay too flat at large radii, and clusters of galaxies contain more gravity than visible matter can provide. Gravitational lensing is especially important because mass bends light, letting scientists map mass even when it is invisible. The evidence became compelling over decades, especially through the 1970s and later large surveys.