Dark Matter Explained

Dark matter isn't "dark" the way a shadow is dark. It doesn't absorb, reflect, or emit any light at all. It interacts through gravity but apparently nothing else, passing right through ordinary matter like a ghost through a wall.

• It makes up about 27% of the universe's total mass-energy
• Ordinary matter — stars, planets, you — accounts for only about 5%
• Every galaxy sits inside a vast dark matter halo that extends far beyond its visible edge
• Without it, galaxies wouldn't have formed; it provided the gravitational scaffolding for ordinary matter to clump around

We still don't know what dark matter is made of.