Scientists NASA's Chandra X-Ray Observatory have discovered the largest number of black holes in a galaxy outside of our own, and they're all right next door, ready to collide with our galaxy in several billion years.

The team published their findings in a study published in The Astrophysical Journal.

The 26 black holes were found in the larger Andromeda galaxy, also known as M31, which lies a mere 2.5 million light-years from us. All 26 are classified in the stellar mass category, putting their masses at around 5 to 10 times that of our sun.

"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 of the study Stephen Murray from Johns Hopkins University and the Harvard-Smithsonian Center for Astrophysics (CfA). "In the case of Andromeda we have a bigger bulge and a bigger supermassive black hole than in the Milky Way, so we expect more smaller black holes are made there as well." 

Identifying black holes is incredibly difficult because, well, they're invisible. Instead, researchers have to look for signs in the space around them. One way is to watch for streams of material from a nearby star beings sucked in. As the material nears the black hole, it heats up and emits radiation.

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

The team looked at over 150 Chandra X-ray observations over 13 years and received help European Space Agency's XMM-Newton X-ray observatory when pinpointing whether these cosmic objects were indeed black holes.

"By observing in snapshots covering more than a dozen years, we are able to build up a uniquely useful view of M31," said co-author Michael Garcia, also from CfA. "The resulting very long exposure allows us to test if individual sources are black holes or neutron stars." 

Intrepid scientists will be able to get a closer look at these black holes when the Andromeda galaxy collides with our Milky Way galaxy in around 4 billion years, creating one massive galaxy.