Primordial Black Holes (PBH) are theoretical byproducts of the extreme forces during the first microseconds of the Big Bang creation of the Universe. But still theoretical--albeit with a credibility that comes with Stephen Hawking's endorsement.
Supermassive Black Holes are quite different, and now that we have pictures, no longer theoretical. This first photo was taken in 2023 by the Event Horizon Telescope, finding the Supermassive Black Hole at the center of the galaxy M87. These are monsters that swallow gasses, asteroids, planets, stars, and galaxies. The diameter of that black spot at the center of M87 is 13,000 times the diameter our Sun. You can't see it because the gravity captures all light.
A PBH can be small as an atomic nucleus. You can't see a PBH because it is so damn small. So small it can pass through the Earth--or even your body. Would we notice? Stephen Hawking calculated a typical "tiny" PBH mass at over a 100 billion metric tons, or about the same mass as a large Earth mountain. But that mass is compressed into a singularity the size of an atomic nucleus! Hawking predicted PBHs eventually die via evaporation in the vacuum of space with a cataclysmic explosion that yields a million megatons. There are multiple observations and theories around the possibilities. Here are a few references (hundreds more can be found with a Web search):
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