Observed: The Day the Universe Lit Up

Although it was born in the fireball brilliance of the Big Bang, the universe spent much of its infancy in the dark. Clouds of primordial particles expanded and cooled forming atoms--hydrogen, mostly that were opaque to light. Galaxies did not yet exist. Even stars did not yet exist. These are known as the cosmic Dark Ages. That era is shrouded in mystery, since scientists literally cannot see what was happening then. Then gravity did its work. Gas collapsed into bright stars, larger clumpings of matter collected into proto-galaxies, and the universe began to light up. Astronomers have worked out the general theory of how this probably happened. Now at last they are seeing it for real, due to data coming from the brand-new Atacama Large Millimeter/submillimeter Array, or ALMA.