"Asteroid-mass black holes, if they were all of the dark matter, might pass through the Earth once a millennium or so, but would be very, very hard to detect," Brandt told Business Insider. "If you had somebody right there, they might be able to observe one."
Brandt was skeptical asteroid-size black holes would be all that dangerous, though.
And if a heavier, sub-moon-size black hole came too close?
"We certainly would notice if one passed near the Earth, since it would affect the orbits of all of our satellites," he wrote in an email. "I imagine that it would mess up GPS for example."
The good news here, says Brandt, is that mini-black holes of this size would pass between Earth and the sun once every 100 million years or so.
"We would, on average, have to wait much longer than the age of the Universe for one to pass through the Earth. Though such an event is absurdly unlikely ... It would cause some havoc," he wrote.
That could definitely kill someone, Brandt noted, since it would be "a bit like a bullet, but with the damage being done by tidal forces deforming the object and generating intense heat."
Yet the scariest scenario — at least to scientists like Brandt and Kashlinsky — is what super-tiny, essentially impossible-to-detect black holes would mean for science.
"It's possible there is no interaction of dark matter [with normal matter] except through gravity," Brandt said. "If that's the case, we're in trouble. We've never come to that point where we know something is out there but is completely invisible to our experiments."
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