HLX-1 has been described as being discovered almost by accident, as the research team at the time was instead focused on its host spiral galaxy. Black holes are generally more likely to sit at the center of galaxies such as the one that is believed to exist at the center of our own Milky Way. But HLX-1 was found, uncharacteristically, out in the spiral. It came to notice only because it was spewing a lot of x-rays and radio flares.
Because of those findings, this new research team began to focus exclusively on the black hole, hoping that it would be the first example found of an intermediate sample. To figure out if it was, the team took measurements from around the time HLX-1 was first discovered and applied theoretical formulas that have been derived over the years to predict the behavior of intermediate black holes. Then, last year, they made a second round of observations and found they matched almost perfectly with what the theories had predicted leading the researchers to proclaim HLX-1 as the first discovered intermediate mass black hole.
How intermediate mass black holes have come to exist is still not very well understood however. Some suggest they may have sprung into existence as tight clusters of stars collapsed into one single black hole. Others theorize that they may have come about as entities all on their own in the aftermath of the big bang; others yet say that maybe they started out as massive black holes that shrunk over time for unknown reasons. Because of the many possibilities, researchers will undoubtedly be kept busy for many years trying to find the best possible explanation, but at least now they will have a real one to study.