By Will Dunham

WASHINGTON (Reuters) - A balloon-borne instrument soaring high over Antarctica has found evidence of a possible large clump of mysterious so-called dark matter relatively close to our solar system, scientists said on Wednesday.

It detected an unexpected amount of very high energy cosmic ray electrons coming from an unknown source within about 3,000 light years of the solar system. A light-year is 6 trillion miles (10 trillion km), the distance light travels in a year.

One explanation is that the electrons may have been spawned as dark matter particles collided with one another, triggering their mutual annihilation, according to Louisiana State University physics professor John Wefel.

Scientists think perhaps 25 percent of the universe is made up of dark matter, which responds to gravity the same way as does regular matter such as stars and planets and the like.

While the stuff is thought to be strewn throughout the cosmos, it is invisible and poorly understood. Scientists have struggled to find any solid evidence of dark matter, and the new study could represent a major step forward in that effort.

"This would be the first indirect detection of the annihilation signature of predicted dark matter particles," Wefel, who helped lead the research published in the journal Nature, said in a telephone interview.

Scientists think regular matter amounts to about 5 percent of the universe's mass. The remaining roughly 70 percent is thought to be composed of dark energy, a mysterious presence that may be making the universe expand at an accelerated pace.

The scientists said it is possible that the electrons detected in this research may have been produced by a more conventional source -- perhaps a spinning neutron star called a pulsar that emits a beam of radiation, a medium-sized black hole or the remnants of a bygone supernova.

Scientists think dark matter is distributed somewhat uniformly throughout the universe, with clumps forming around concentrations of regular matter -- for example, galaxies -- due to their gravitational pull.

The scientists think the electrons detected by the instrument may come from one of these clumps located relatively close in astronomical terms to our solar system.

"If our data is to be explained this way, there's got to be some sort of a clump," Wefel said.

The electrons detected by the instrument seem to match theories about what would be produced when dark matter particles collide and destroy each other.

"The annihilation of these exotic particles with each other would produce normal particles such as electrons, positrons, protons and antiprotons that can be observed by scientists," Eun-Suk Seo of the University of Maryland said in a statement.

The NASA-funded instrument was carried to an altitude of about 24 miles above Antarctica using a helium-filled balloon as big as the interior of a large sports stadium.