A plethora of black holes have been discovered in the Andromeda Galaxy, one of the galaxies nearest to the Milky Way, a U.S. researcher said.

Using data from more than 150 observations by NASA's X-ray Observatory spread over a 13-year period, researchers identified 26 black hole candidates in Andromeda, said Robin Barnard of Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and lead author of a paper describing the results.

"While we are excited to find so many black holes in Andromeda, we think it's just the tip of the iceberg," Barnard said Wednesday in a release. "Most black holes won't have close companions and will be invisible to us."

Astronomers can detect these otherwise invisible objects as material is drawn away from a companion star and heated to produce radiation before it disappears into the black hole.

The research group previously identified nine black hole candidates within the region covered by the Chandra data, so the present results increase the total to 35, the center said. This differentiates Andromeda from the Milky Way because astronomers have yet to find a similar black hole in one of the Milky Way's globular clusters.

"When it comes to finding black holes in the central region of a galaxy, it is indeed the case where bigger is better," said co-author Stephen Murray of Johns Hopkins University and Center for Astrophysics.

Results are available online and will be published in the June 20 issue of The Astrophysical Journal.