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martyb writes:

Dark Matter is Not Made Up of Tiny Black Holes:

An international team of researchers has put a theory speculated by the late Stephen Hawking to its most rigorous test to date, and their results have ruled out the possibility that primordial black holes smaller than a tenth of a millimeter make up most of dark matter. Details of their study have been published in this week's Nature Astronomy.

Scientists know that 85 per cent of the matter in the Universe is made up of dark matter. Its gravitational force prevents stars in our Milky Way from flying apart. However, attempts to detect such dark matter particles using underground experiments, or accelerator experiments including the world's largest accelerator, the Large Hadron Collider, have failed so far.

This has led scientists to consider Hawking's 1974 theory of the existence of primordial black holes, born shortly after the Big Bang, and his speculation that they could make up a large fraction of the elusive dark matter scientists are trying to discover today.

Original Submission

The scientists theorized that primodial black holes between Earth and the Andromeda galaxy. Were one to lie between us and a star, then it would cause the star to appear to brighten for a few minutes or hours.

From 190 images of the Andromeda galaxy taken over the course of a single, 7-hour observation, the researchers expected to see about 1000 events. They saw... just one. They haven't given up trying to locate the "missing mass" — mass we have not yet identified, but would be needed to explain the orbital properties of galaxies. To wit:

The researchers are now planning to further develop their analysis of the Andromeda galaxy. One new theory they will investigate is to find whether binary black holes discovered by gravitational wave detector LIGO are in fact primordial black holes.

Journal Reference:
Hiroko Niikura, et. al. Microlensing constraints on primordial black holes with Subaru/HSC Andromeda observations. Nature Astronomy, 2019; DOI: 10.1038/s41550-019-0723-1