https://hackerpaper.kaipfister.de/p/1701.02151/Collected HN comments and submissions linking this paper.enMon, 02 Nov 2026 11:16:22 GMThttps://news.ycombinator.com/item?id=23093982urn:hackerpapers:mention:1701.02151:23093982Wed, 06 May 2020 17:41:06 GMT<p>We have! 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&#x27;t dark matter at this time, the under/overdensities of baryons alone are not large enough to produce the large scale structure (galaxy groups/clusters) that we see today. 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. This…</p><p><a href="https://hackerpaper.kaipfister.de/p/1701.02151/">Paper and discussion history</a>. A mention is not an endorsement.</p>astro-ph.CO