These galaxies, one of which appears to have a mass rivaling our Milky Way but 30 times more densely packed, seem to differ in fundamental ways from those populating the universe today. "The leading theory is that an ocean of dark matter filled the early universe after the Big Bang," Labbe said. "This dark matter - we don't know what it is actually is - started out really smooth, with only the tiniest of ripples. These ripples grew over time due to gravity and eventually the dark matter started to collect in concentrated clumps, dragging hydrogen gas along for the ride. "Their explosion set off the chain of events that formed subsequent generations of stars," Labbe said.

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