The noise, picked up by a balloon-borne instrument, makes no sense, based on science's current understanding of the cosmos, the scientists at the National Aeronautics and Space Administration said.

"The universe really threw us a curve," said astrophysicist Alan Kogut of NASA's Goddard Space Flight Center in Greenbelt, Md.

"Instead of the faint signal we hoped to find, here was this booming noise six times louder than anyone had predicted," he said.

Scientists ruled out primordial stars or known radio sources -- including gas in the outermost halo of our own galaxy, the Milky Way -- as the noise's source.

But they don't know anything else about it, including its cause or why it's so loud.

The instrument, launched in July 2006 from NASA's Columbia Scientific Balloon Facility in Palestine, Texas, flew to an altitude of 120,000 feet, where the atmosphere thins into the vacuum of space.

Its mission was to search the sky for heat from the first generation of stars. Instead, it found a cosmic puzzle, NASA said.

The noise complicates NASA's efforts to detect the first stars, thought to have formed about 13 billion years ago -- not long, in cosmic terms, after the Big Bang.