Dark matter makes up roughly , dwarfing the "ordinary" matter—stars, planets, and people—which accounts for less than 5%.

Scientists discovered dark matter not by seeing it, but by noticing that the universe's "math" didn't add up without it. Galaxy Rotation Curves

In the 1970s, astronomer Vera Rubin observed that stars at the edges of spiral galaxies were moving just as fast as those near the center. According to Newtonian physics, they should have been moving much slower or flying off into space unless some unseen mass was holding them in place. Galaxy Clusters

Most evidence suggests dark matter is "cold," meaning its particles move slowly relative to the speed of light. 🔭 Key Evidence