Massive galaxies that formed and stopped producing stars soon after the Big Bang were created by violent star formation triggered by the collision of galaxies, a new study published in the Astrophysical Journal argues.

According to the researchers, the universe's structure was formed as small, baby galaxies collided with neighboring galaxies and produced new stars of their own. If true, this would mean that the largest galaxies seen today have been under construction since the beginning of the universe.

"That is why it surprised us that we already, when the universe was only 3 billion years old, found galaxies that were just as massive as today's large spiral galaxies and the largest elliptical galaxies, which are the giants in the local universe," Sune Toft, from the Dark Cosmology Center at the Niels Bohr Institute at the University of Copenhagen, said in a statement.

Toft and his colleagues were further surprised to find that the stars in these galaxies were all stuffed into a small region, "so the size of the galaxies were three times smaller than similar mass galaxies today. This means that the density of stars was 10 times greater."

What's more, the galaxies were asleep, their star formation stopped.

"It was a great mystery," he said.

Toft knew the galaxies formed early in the universe's history - so early, in fact, that they would not have had the chance to grow as big as they did through normal star formation. One possibility, he thought, was that they developed when smaller galaxies fused together. Still, this was not enough by itself to explain their quick growth or why they were dead.

"We studied the galaxies that existed when the universe was between 1 and 2 billion years old," Toft said. "My theory that it must have been some galaxies with very specific properties that were part of the formation process made me focus on the special SMG galaxies, which are dominated by intense star formation hidden under a thick blanket of dust."

Toft explained that when gas-rich galaxies merge, their gas gets pushed into the middle, causing star formation so intense, the resources are quickly used up, leaving the galaxy dead.

The researcher said: "I discovered that there was a direct evolutionary link between two of the most extreme galaxy types we have in the universe - the most distant and most intense star forming galaxies which are formed shortly after the Big Bang - and the extremely compact dead galaxies we see 1-2 billion years later."