No massive black holes in the Milky Way halo.

The gravitational wave detectors have shown a population of massive black holes that do not resemble those observed in the Milky Way and whose origin is debated. According to a possible explanation, these black holes may have formed from density fluctuations in the early Universe (primordial black holes), and they should comprise several to 100% of dark matter to explain the observed black hole merger rates. If these black holes existed in the Milky Way dark matter halo, they would cause long-timescale gravitational microlensing events lasting years.

A three-dimensional map of the Milky Way using classical Cepheid variable stars.

The Milky Way is a barred spiral galaxy, with physical properties inferred from various tracers informed by the extrapolation of structures seen in other galaxies. However, the distances of these tracers are measured indirectly and are model-dependent. We constructed a map of the Milky Way in three dimensions, based on the positions and distances of thousands of classical Cepheid variable stars.