That leaves scientists wondering how the tiny, star-rich galaxies formed in the first place.
It's possible that in the uneven landscape of the early universe, pockets of dark matter trapped pools of hydrogen gas, which began spinning rapidly and forming stars at a breakneck pace.
Such events "could be quite spectacular," van Dokkum said. "They had to form a lot of stars in a very small [volume] during a very short time."
The find also raises questions about whether the tiny young galaxies contributed to the growth of larger galaxies that exist today.
One theory suggests that the dense galaxies eventually puffed up through collisions and mergers. (See images of galaxies colliding.)
But it's unclear whether this kind of growth could have swelled the objects by a factor of five or six to reach the size of present-day galaxies.
"Could galaxies really decrease that much in density by adding new gas and stars?" asked Eric Gawiser, a cosmologist at Rutgers University who was not involved with the research.
"Or could it be that this team has overestimated the densities in the past due to the difficulty of the measurement?
"One sign that the [team's] result is correct is that the average density of matter in the universe was ten times greater when these ultradense galaxies formed," Gawiser said.
"So everything was denser when the universe was younger."
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