TORONTO -- The oldest, largest galaxies in the universe once lived fast and furiously, according to astronomers, forming stars in a remarkably short amount of time. But about three billion years after the Big Bang, they hit the brakes and the growth of these galaxies stalled.

"The most massive galaxies in our universe formed incredibly early, just after the Big Bang happened, 14 billion years ago," Kate Whitaker, professor of astronomy at the University of Massachusetts Amherst, said in a news release. "But for some reason, they have shut down. They're no longer forming new stars."

It's a mystery that has puzzled experts, until now.

Turns out these galaxies have been running out of the cold gas needed to form new stars.

In a study published Wednesday in the journal Nature, astronomers observed some of these ancient, distant galaxies. These galaxies are so far away that we're only just now seeing light they emitted 10 to 12 billion years ago, which is when the universe was in its infancy.

In effect, it's a look into the deep past.

"There was copious cold gas in the early universe, so these galaxies, from 12 billion years ago, should have plenty left in the fuel tank," Whitaker said.

But, it turns out, that isn't the case.

Whitaker's team, by combining images from the Hubble Space Telescope and readings from the Atacama Large Millimeter/submillimeter Array, which detects radiation invisible to the naked eye, found only trace amounts of cold molecular gas at the centre of these galaxies.

This means that these galaxies either burned through their energy supplies or ejected them within the first few billion years of the universe's existence, according to the researchers. It also raises the possibility that something may be blocking each galaxy's replenishment of cold gas.

Next, the research group intends to investigate how compact the remaining cold gas is in these inactive and dormant galaxies and why it exists only in the centre of these galaxies.