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Primordial Galaxies Swimming in Vast Ocean of Dark Matter
Artist impression of a pair of galaxies from the very early universe. Ongoing observations with the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered surprising examples of massive, star-filled galaxies seen when the cosmos was less than a billion years old. This suggests that smaller galactic building blocks were able to assemble into large galaxies quite quickly. The latest ALMA observations push back this epoch of massive-galaxy formation even further by identifying two giant galaxies seen when the universe was only 780 million years old, or about 5 percent its current age. ALMA also revealed that these uncommonly large galaxies are nestled inside an even-more-massive cosmic structure, a halo of dark matter several trillion times more massive than the sun.
On a clear, dark night, you can see a glowing stream that seems to split the sky. We have called it the Milky Way for thousands of years, and its exact nature was a mystery until less than a hundred years ago.
