Copenhagen, Jan 30 (IANS): Enormously massive galaxies that were already old and no longer forming new stars in the very early universe, about three billion years after the Big Bang, have long puzzled scientists -- fresh research has finally provided the answer to the mystery.

New research shows that these massive galaxies were formed by explosive star formations that were set in motion by the collision of galaxies a few billion years after the Big Bang.

"It surprised us that we, already when the universe was only three 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,” said Sune Toft at the Niels Bohr Institute, University of Copenhagen, Denmark.

“Even more surprisingly, the stars in these early galaxies were squeezed into a very small area, so the size of the galaxies was three times smaller than similar mass galaxies today,” Toft added.

This means that the density of stars was 10 times greater. Furthermore, the galaxies were already dead, so they were no longer forming new stars.

It was a great mystery -- how could the galaxies become so massive and so burnt out so early?

To find the answer, Toft had to look even further back in time.

“We studied the galaxies that existed when the universe was between one and two 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 galaxies, which are dominated by intense star formation hidden under a thick blanket of dust," explained Toft.

The smallest galaxies contain a few million stars, while the largest can contain several hundred billion stars.

The first stars already emerged in the very early universe approximately 200 million years after the Big Bang from hydrogen and helium gases, said the study that was published in the Astrophysical Journal.