3. Dark Matter
Dark matter is thought to make up roughly 27% of the universe, yet its composition and properties remain shrouded in mystery. Scientists can observe its gravitational influence on galaxies and cosmic structures, but direct detection has proven elusive. Theories suggest exotic particles like WIMPs or axions might be responsible, but so far, these remain hypothetical. The quest to understand dark matter continues to challenge physicists and astronomers alike. Explore further at NASA.