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New Research Reveals Hundreds of Undiscovered Black Holes (phys.org)
An anonymous reader quotes a report from Phys.Org: New research by the University of Surrey published in the journal Monthly Notices of the Royal Astronomical Society has shone light on a globular cluster of stars that could host several hundred black holes, a phenomenon that until recently was thought impossible. Globular clusters are spherical collections of stars which orbit around a galactic center such as our Milky-way galaxy. Using advanced computer simulations, the team at the University of Surrey were able to see the un-see-able by mapping a globular cluster known as NGC 6101, from which the existence of black holes within the system was deduced. These black holes are a few times larger than the Sun, and form in the gravitational collapse of massive stars at the end of their lives. It was previously thought that these black holes would almost all be expelled from their parent cluster due to the effects of supernova explosion, during the death of a star. It is only as recently as 2013 that astrophysicists found individual black holes in globular clusters via rare phenomena in which a companion star donates material to the black hole. This work, which was supported by the European Research Council (ERC), has shown that in NGC 6101 there could be several hundred black holes, overturning old theories as to how black holes form.
http://arxiv.org/abs/1609.0172...
From the article
"This research is exciting as we were able to theoretically observe the spectacle of an entire population of black holes using computer simulations. The results show that globular clusters like NGC 6101, which were always considered boring are in fact the most interesting ones, possibly each harbouring hundreds of black holes. This will help us to find more black holes in other globular clusters in the Universe. " concluded Peuten.
Therefore still undiscovered, as this was just a simulation that provided an explanation of the makeup of the specific cluster under examination - when they added black holes to the simulation, the results matched what the actual cluster looks like while without black holes it didn't
Some novae occur though a gentle accretion process, such as a burnt out star with a companion, and some occur through expending all of thier fuel and the lack of thermal pressure causes collapse. Rarely one can be created through a merger of two stars but it's not exactly the same thing. In the first two cases it's not very clear what causes this but there is some evidence the process is not always symmetrical. A supernova often expends as much energy over a few hours to days as the star previously expended in its lifetime. Much of the material is blown off at up to 10% light speed. With the release of so much energy, even a very small deviation from being spherical can provide enough velocity on the black hole to exceed the pull of the gravity in these small globular clusters. So the result of the simulation is interesting and points the way forward for follow up observation.
Not really, that's just one of the major theories for it's nature, that it's WIMPs - Weakly Interacting Massive Particles. Another popular theory is that it's MACHOs - Massive Compact Halo Objects, aka black holes orbiting outside the radius of the visible galaxy - a theory that has gained credence with the recent detection of two black whole mergers, both within the narrow size window which would remain consistent with observations to date, suggesting that such black holes are far more common than we believed. And there's several other theories as well - so far none has truly compelling evidence to separate it from the pack.
And of course, there's no guarantee that dark matter exists at all - it's basically just a "here there be dragons" kludge of current gravitational theory, necessary to make predictions consistent with observed reality. As we revise or replace General Relativity we may discover that the need for dark matter vanishes as well. You say that Newton could not say what gravity really was - today physicists realize that they understand it even less than he thought he did.
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