<story><title>ESO Instrument Finds Closest Black Hole to Earth</title><url>https://www.eso.org/public/news/eso2007/</url></story><parent_chain><item><author>ifdefdebug</author><text>From the article: «The discovery of a silent, invisible black hole in HR 6819 provides clues about where the many hidden black holes in the Milky Way might be. “There must be hundreds of millions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them,” says Rivinius. Baade adds that finding a black hole in a triple system so close by indicates that we are seeing just “the tip of an exciting iceberg.”»&lt;p&gt;As a layman I wonder if this could be the solution for dark matter ... but experts probably already checked that idea.</text></item></parent_chain><comment><author>astro123</author><text>We have!&lt;p&gt;A constraint on dark matter is that it needs to be around at very early times. Before the time of the CMB (400 000 years after the Big Bang). If there wasn&amp;#x27;t dark matter at this time, the under&amp;#x2F;overdensities of baryons alone are not large enough to produce the large scale structure (galaxy groups&amp;#x2F;clusters) that we see today.&lt;p&gt;What this means is that black holes, formed in the conventional process (stars dying), cannot be dark matter. The first stars only formed much later. However, primordial black holes (formed at very early times, before the CMB) were still a possibility.&lt;p&gt;This possibility has mostly been ruled out though. The main way we have done that is through microlensing. If there were lots of reasonably sized black holes floating around, they would magnify background stars as the passed in front of them. It&amp;#x27;s a pretty cool effect. Here&amp;#x27;s a nature paper from a couple of years ago that investigated it [1]. The abstract is very readable and figure 5 shows how people have been slowly ruling out black holes as a major component of DM.&lt;p&gt;[1] &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;arxiv.org&amp;#x2F;pdf&amp;#x2F;1701.02151.pdf&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;arxiv.org&amp;#x2F;pdf&amp;#x2F;1701.02151.pdf&lt;/a&gt;</text></comment>
<story><title>ESO Instrument Finds Closest Black Hole to Earth</title><url>https://www.eso.org/public/news/eso2007/</url></story><parent_chain><item><author>ifdefdebug</author><text>From the article: «The discovery of a silent, invisible black hole in HR 6819 provides clues about where the many hidden black holes in the Milky Way might be. “There must be hundreds of millions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them,” says Rivinius. Baade adds that finding a black hole in a triple system so close by indicates that we are seeing just “the tip of an exciting iceberg.”»&lt;p&gt;As a layman I wonder if this could be the solution for dark matter ... but experts probably already checked that idea.</text></item></parent_chain><comment><author>throwaway2048</author><text>The idea that dark matter is hidden inside black holes has been ruled out via observations looking for gravitational micro-lensing† which is basically looking for distortions in our views of the sky. Although we cant easily locate specific black holes, we know there is not enough micro lensing going on for the explanation for dark matter to possibly be black holes (the total mass of the black holes would have to be many times more than the amount of visible matter in the universe)&lt;p&gt;† &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Gravitational_microlensing&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Gravitational_microlensing&lt;/a&gt;</text></comment>