Astronomers have long assumed there were far more so-called "active" black holes than had been observed, but were unable to find any trace of them.

An international team of astronomers unexpectedly found hundreds of expanding "supermassive" black holes buried deep inside galaxies billions of light years from earth. The findings more than double the total number of black holes known to exist at that distance, and suggest there were hundreds of millions more growing in the early universe.

These super-massive entities are known as high-energy quasars, a form of black hole, found in a young galaxies, that is surrounded by a thick halo of gas and dust which shoot off X-rays as they are sucked into the void.

The X-rays, which can be detected as a general glow in space even when the quasars themselves cannot be seen, are what tipped off the scientists that they had stumbled across something extraordinary.

The astounding discovery is the first direct evidence that most - perhaps all - huge galaxies in the far reaches of the universe generated cavernous black holes during their youth, when about 3.5 billion years old.

"We had seen the tip of the iceberg before in our search for these objects. Now, we can see the iceberg itself," said Mark Dickinson of the National Optical Astronomy Observatory in Tucson, Arizona.

"We knew from other studies from about 30 years ago that there must be more quasars in the universe, but we didn't know where to find them until now," said French astrophysicist Emanuel Daddi, who led the research. Daddi and his team set out to study about 1,000 galaxies - about the same mass as the Milky Way - in the process of making stars, but thought to lack quasars.

At nine to 10 billion light years distant, what scientists see today existed about 10 billion years ago, when the universe was still a fledgling between 2.5 and 4.5 billion years old. The quasars will help answer fundamental questions about how massive galaxies evolve. Astronomers now know, for example, that most of these galaxies steadily generate stars and black holes simultaneously until the latter become too big and impede star formation.

Two telescopes were needed to see the black holes. One is NASA's Spitzer space telescope, which picks up infrared light, and the other is the Chandra telescope, which relays X-ray data.

The findings will be published next month in the US journal Astrophysical Journal.

Posted by Casey Kazan. Adapted from a Science Daily research release.