Astronomers have detected faint X-ray signals that they believe were emitted by giant black holes billions of years ago when the Universe was a toddler, a study in the journal Nature said on Wednesday.

The probe looked at more than 250 galaxies, the farthest of which was 13 billion light years from Earth -- in other words, emissions that came only 700 million years or so after the "Big Bang" that created the cosmos.

The findings support the theory that super-sized black holes play a dual role of destruction and creation at the heart of galaxies, its authors said.

"These observations indicate that extremely massive black holes already existed as early as 700-800 million years after the Big Bang, which suggests that either they were born massive to start with, or they experienced rapid growth bursts," said Yale University cosmologist Priyamvada Natarajan.

"Either scenario tells us much more than we previously knew, which is very exciting."

The black holes were discovered indirectly, thanks to telltale high-energy X-ray signals which escaped through thick clouds of dust and gas in their host galaxies.

Ancient black holes have already been spotted in previous research, but this is the first wide-scale trawl to look across a range of galaxies.

Interesting-looking galaxies were selected from a scan of the skies by the Hubble Space Telescope. The search was then finetuned using another NASA orbital telescope, the Chandra X-Ray Observatory.