By BBC News Online Science Editor Dr David Whitehouse

The Chandra X-ray observatory orbiting the Earth has solved a long-standing puzzle of modern astronomy - the source of the X-rays that bathe the Universe in a mysterious high-energy glow.

Ever since astronomers started using X-rays to probe the hot and violent parts of the Universe, they have wondered about the so-called X-ray background.

X-rays come from hot gas, usually heated to millions of degrees by falling into a black hole or scattered into space by a stellar explosion. They do not penetrate the Earth's atmosphere so satellites must be used to detect them.

"This is a major discovery," said Dr Alan Bunner, Director of Nasa's Structure and Evolution of the Universe science research group. "Since it was first observed 37 years ago, understanding the source of the X-ray background has been a 'Holy Grail' of X-ray astronomy. Now, it is within reach."

The results of Chandra's observations were presented to the annual meeting of the American Astronomical Society in Atlanta, US.

"We are all very excited by this finding," said Dr Richard Mushotzky of Nasa's Goddard Spaceflight Center. "The resolution of most of the X-ray background during the first few months of the Chandra mission is a tribute to the power of this observatory and bodes extremely well for its scientific future."

70 million sources

To solve the mystery, the Chandra team looked at a small section of the sky, a circle about one-fifth the size of a full moon. They stared for 27.7 hours and resolved about 80% of the mysterious X-ray glow in this region into discrete X-ray sources.

Stretched across the entire sky, this adds up to approximately 70 million sources, most of which are galaxies radiating X-rays. One-third of the sources are galaxies whose cores shine bright in X-rays, yet do not shine in visible light.

According to the astronomers, there may be tens of millions of these "veiled galactic nuclei" in the Universe. Each of them probably harbours a massive black hole at its core that produces X-rays as gas is pulled toward it at nearly the speed of light.

A second new class of objects has been observed in the image. Comprising approximately one-third of the sources, they are thought to be "ultra-faint galaxies".

Dr Mushotzky said that these sources may emit little or no optical light, either because the dust around the galaxy blocks the light totally or because the optical light is eventually absorbed during its long journey across the Universe.

He added that these sources would be well over 14 billion light years away and thus the earliest, most distant objects ever identified.