This blog has always emphasised that there is more in the Universe than meets the eye. Theory predicts, and observations confirm that baryonic matter is only 4.507034% of the total mass. Flying over a city at night, the lights we see are just a hint of what lies beneath. The matter we see is only a small fraction of what is out there.
Object VIRGOH121 was first noticed in 2000 by its influence on a nearby galaxy. NGC 4254 has a huge stream of matter torn from its side. It must have suffered a collision with an object of 100 billion solar masses. When the most powerful telescopes looked in this region they saw nothing. VIRGOH121 contains no stars. Only by its radio emissions was it located.
When this research was first published, the astronomy community was very skeptical. Alternative theories were cooked up to explain the observations. Perhaps they invoked some sort of "dark energy." Now, 7 years after first publication, there is conclusive evidence that VIRGOH121 is a dark galaxy. It is entirely made up of dark mass, which could be Black Holes.
We are fortunate to have found VIRGOH121 because of its collision with a visible galaxy. There could be many, many more dark galaxies out there. If the dark mass is composed of Black Holes, they would have formed shortly after the Big Bang and never have been matter. Dark mass could be everywhere, even within our solar neighbourhood.
The next time you look up at the stars, try to imagine a parallel Universe occupting the same Space/Time. this Universe is hidden from our eyes, yet contains 20 times as much mass. Interactions with this dark mass created galaxies, stars and even planets. Our understanding of the Universe is just beginning.