Its not every day that astronomers get to discover an unexpected and baffling new class of object in the universe. NASAs powerful Chandra X-ray space observatory has provided definitive evidence of a completely new type of black hole residing 10 billion light-years away in the peculiar galaxy M 82, located in the constellation Ursa Major.

Weighing in at no less than 500 times the mass of our sun, it has been dubbed a "missing link" because it is much larger than that the three-to-20-solar-mass black holes created by individual stellar explosions. However it is also much smaller than the gargantuan million- or even billion-solar-mass black holes that lurk at the centers of many galaxies.

Chandra showed that the black hole is not at the exact center of the galaxy, so it is not a classic active galactic nucleus where a monster black hole feeds on gas. Chandra also found the black hole fluctuates over a long enough period that it must have an immense disk of glowing gas unlike any seen before in objects other than a supermassive black hole.

A survey of 39 nearby galaxies with the German -U.S.-U.K. ROSAT (Rontgen Satellite) X-ray satellite, by Ed Colbert of the Johns Hopkins University (JHU) and Richard Mushotzky of NASAs Goddard Space Flight Center, reveal at least three other mid-sized black holes that are also offset from the nucleus of their respective galaxies, with mass estimates ranging from 117 to 8,500 times the suns mass.

Top black hole experts confess they are baffled.

"The whole thing is a mystery". Said black hole expert Roeland Van Der Marel of Space Telescope Science Institute in Baltimore Maryland.

"We don't have a good reason why these black holes are here,"  adds Colbert. "There is no simple scenario that completely explains how to form an intermediate-mass black hole."

Speculation has ranged from the middleweight black hole being primordial -- forged in the dim early days of the universe through unknown processes -- or a remnant collapsed core of a disintegrated star cluster.

Richard Griffiths of the Carnegie-Mellon University in Pittsburgh has offered one of the most intriguing explanations. He believes the mid-size black hole may really be in a transitory, developmental phase between stellar and galactic black holes. "We may be seeing a snapshot of an adolescent black hole on its way to growing up to a full adult supermassive black hole," said Griffiths.