Galaxy rotation curves contradict visible matter
Galaxies rotate with a velocity curve that violates Newtonian mechanics if only visible matter is counted. Outer stars should move slower than inner stars (like planets in our solar system), yet they maintain near-constant velocity. The only explanation is additional invisible mass throughout the galaxy, contributing gravitational pull without emitting light. This discrepancy has held since the 1930s and is the most straightforward evidence for dark matter.
Rotation curves also reveal a dark matter halo extending far beyond the visible disk, a spherical envelope of dark matter dwarfing the spiral structure we see in telescopes. The ratio of dark to visible matter varies by galaxy type but averages 5-10:1 for spiral galaxies.
Gravitational lensing, CMB peaks, and clustering
When massive clusters bend light from distant galaxies, the bending angles reveal the total cluster mass through general relativity. Comparing visible light from galaxies to the lensing strength shows that most mass is dark. Similarly, the cosmic microwave background's temperature power spectrum (the height of peaks at different angular scales) directly constrains the dark matter density. The peak heights depend on how much matter was present 380,000 years ago; observations fix dark matter at 27 percent of the universe's current density.
Galaxy clustering on large scales (hundreds of millions of light-years) shows structure that matches computer simulations only when dark matter is included. Without it, simulations produce too-clumpy, too-sparse structures that do not match the observed cosmic web.