The nature of the universe recently became a little clearer, as astronomers from Durham University announced the discovery of a large population of supermassive black holes that were previously hidden from view by gas and dust. Using data from NASA's NuSTAR satellite observatory, the researchers confirmed the detection of high-energy x-rays from black holes that could be billions of times the mass of the Sun.
Supermassive black holes are firm residents of the "mysterious" category of the universe, yet they are also a major clue in our understanding of the structure and evolution of large galaxies. As the name implies, supermassive black holes are black holes with masses thousands to billions times that of the Sun. They're almost invariably found at the centers of massive galaxies, where they form and grow through the accretion of gases that spiral into their event horizons, where the laws of normal space no longer apply.
Because of their size, such black holes have a density less than that of water and no significant tidal forces. They also generate high-energy x-rays due to the incoming gases speeding up and crowding together. This is particularly important because this is how astronomers detect them. Because of their strange properties, black holes cannot be seen directly. 
The existence of such giant black holes was suspected since 1971. The first detected was Sagittarius A, which was discovered in 1974. It's located in the center of our Milky Way galaxy, some 26,000 light years from Earth. Since then, only a handful of other supermassive black holes have been detected in other galaxies, so their numbers and natures have remained a puzzle. 
The key to the Durham University project was NASA's Nuclear Spectroscopic Telescope Array (NuSTAR). Launched in 2012, the space telescope is the 11th in NASA’S small explorer satellite program and the first direct-imaging orbital x-ray telescope operating at energies above those of the Chandra X-ray Observatory and XMM-Newton. According to the Durham team, it is in these high-energy x-ray bands that the evidence for supermassive black holes is found.
The evidence from NuSTAR, which was presented on Monday at the Royal Astronomical Society’s (RAS) National Astronomy Meeting in Llandudno, Wales, indicates the existence of five supermassive black holes at the center of other galaxies (which were previously obscured from view) out of nine candidates selected for study by the orbital x-ray telescope. 
A Hubble Space Telescope colour image of one of the nine galaxies targeted by NuSTAR
According to the researchers, the newly-found black holes are more active than previously thought. Their discovery is particularly important because it allows astronomers to extrapolate the population of such black holes in the universe, which they estimate could run into the millions
"For a long time we have known about supermassive black holes that are not obscured by dust and gas, but we suspected that many more were hidden from our view," says George Lansbury, a postgraduate student in the Centre for Extragalactic Astronomy, at Durham University. "Thanks to NuSTAR for the first time we have been able to clearly see these hidden monsters that are predicted to be there, but have previously been elusive because of their 'buried' state ... Although we have only detected five of these hidden supermassive black holes, when we extrapolate our results across the whole Universe, then the predicted numbers are huge and in agreement with what we would expect to see."
The team's results were published in The Astrophysical Journal.
Source: RAS