dark matter halo

Every galaxy you see is the visible tip of something far larger and completely invisible. Surrounding the bright disk or ball of stars is a vast, roughly spherical cloud of dark matter — material that gives off no light at all and is detected only by its gravity. This enveloping cloud is called the dark matter halo, and the stars and gas we see sit deep inside it like the yolk inside an egg.

Dark matter is some substance that has mass and therefore pulls on things by gravity, but does not emit, absorb, or reflect light, and apparently does not collide with itself or with normal matter except gravitationally. A galaxy's halo typically outweighs all its stars and gas by a factor of five to ten and extends several times farther than the visible galaxy. We map halos by their gravitational effects: the flat rotation curves of spirals, the high random speeds of stars in ellipticals, the way clusters bend background light through gravitational lensing, and the motions of satellite galaxies all reveal mass that no starlight accounts for.

Halos matter because, in modern cosmology, they are the scaffolding on which galaxies are built. In the leading cold-dark-matter picture, dark-matter halos formed first, growing by merging smaller halos into bigger ones (hierarchical assembly); ordinary gas then fell into these gravitational wells, cooled, and lit up as stars. So the galaxy is the glowing decoration; the halo is the structure. Be honest about the limit, though: 'dark matter' is a name for a gravitational effect we observe, not a particle anyone has yet caught in a detector.