Astronomers got their first hints that the universe is filled with some invisible, mysterious, massive substance back in the 1930’s—something that must be there and holding things together gravitationally, otherwise the rotation of galaxies would cause them to spin apart. Even now, nobody knows for sure what the mystery stuff is. The leading candidate for the past decade or two has been some sort of exotic elementary particle, forged in the Big Bang—but so far, despite plenty of searching, such a particle has never actually been found.
That means there’s still hope for a dark horse in the dark matter sweepstakes: black holes are certainly dark, and there could be lots and lots of them floating around that we haven’t noticed. But the hope that they’re the answer to the riddle has faded recently with a couple of new papers—one based on what is effectively a thought experiment, the other on an ingenious set of observations with the Kepler space telescope, which was launched in 2009 to search for exoplanets, or worlds orbiting other stars.
One kind of particularly small black hole was already off the table. Such things could have been created in the violent turbulence of very early universe, but would have long since evaporated (some people feared one might be created by the Large Hadron Collider when it switched on in 2008, and go on to swallow the Earth, but since you’re reading this, it didn’t).
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