Discovery of a swarm of stellar-mass black holes around the center of the
Milky Way galaxy. The swarm likely formed as the black holes gradually
decelerated in their orbits and drifted toward the center of the Galaxy.
The discovery of two ultrabright X-ray sources near the center of the galaxy M82,
which could be black holes with masses several hundred times that of the Sun.
Black holes like these may be the seeds for supermassive ones found at
the centers of galaxies.
Discovery of dozens of black holes and neutron stars strung out across tens of
thousands of light years around an elliptical galaxy, as the remains of a collision
between galaxies a few billion years ago.
Discovery of an excess of X-ray sources in the outer reaches of the
Sombrero and other galaxies. The origin of this excess is still unknown.