At the center of most large galaxies, our own Milky Way included, are extremely dense black holes that have masses hundreds of millions times that of the Sun.
Called quasars, these massive black holes are the most radiant objects in the universe, outshining even the brightest galaxies. While the black holes themselves are undetectable, friction and heat from the swirling matter they ingest emit huge amounts of radiation that can be detected by radio telescopes.
This figure shows two Hubble images of quasars from a sample of 20 relatively nearby quasars. Quasar HE0450-2958 (left) appears to lack a host galaxy unlike HE1239-2426 (right), which resides inside a normal host galaxy that displays large spiral arms.