Danish scientists say cosmic collisions created enormously massive galaxies already old and no longer forming new stars in the very early universe.

Astronomers have long been puzzled by the existence of such galaxies just 3 billion years after the Big Bang.

Researchers at the Niels Bohr Institute at the University of Copenhagen have determined these massive galaxies were formed by explosive star formation set in motion by the collision of galaxies not long after the Big Bang, the university reported Wednesday.

The first stars already emerged in the very early universe around 200 million years after the Big Bang from the gases hydrogen and helium, and astronomers say they believe the structure of the universe was built by baby galaxies gradually growing larger and more massive by constantly forming new stars and by colliding with neighboring galaxies to form new, larger galaxies.

That suggests the largest galaxies in today's universe have been under construction throughout the history of the universe, they said.

"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," institute researchers Sune Toft said.

"Furthermore, the galaxies were already dead, so they were no longer forming new stars. It was a great mystery," he said.

Toft said he believed there must have been some especially extreme galaxies involved in the formation process.

"We studied the galaxies that existed when the universe was between 1 and 2 billion years old. 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 (submillimeter) galaxies, which are dominated by intense star formation hidden under a thick blanket of dust."

Such galaxies quickly ate through their gas reserves in around 40 million years and stopped star formation when the universe was still relatively young, he said.

Large galaxies were formed by collisions of these small "baby" galaxies, but did not create further stars.

"Just 3 billion years after the Big Bang we see that half of the most massive galaxies have already completed their star formation," he said.