Lowest Mass Black Hole Yet
Short Version: The recently observed gravitational wave event GW170817 is thought to have involved the merger of two neutron stars. A critical question was whether a super-massive neutron star or an unusually low mass black hole was formed in the merger. New X-ray evidence just reported strongly suggests that a black hole with a mass of 2.7 solar mass was the end product of this event. That black hole would represent the lowest mass black hole observed to date.
Longer version: https://phys.org/news/2018-05-gravitational-event-creation-black-hole.html .
Full paper: David Pooley et al. GW170817 Most Likely Made a Black Hole, The Astrophysical Journal (2018). DOI: 10.3847/2041–8213/aac3d6 , https://arxiv.org/abs/1712.03240
Although this black hole is considered to have an unusually low mass, Discrete Scale Relativity predicts that a 2.7 solar mass black hole is still in the high-mass tail of the stellar-mass black hole population, which peaks in the 0.15 to 0.58 solar mass range. This prediction is consistent with the MACHO microlensing results.
I am confident that future gravitational wave research will eventually solve the dark matter mystery.