When the idea of dark matter first pushed its way into astronomers’
consciousness a few decades ago, the primary reaction was: “Seriously?
There’s a mysterious, invisible substance out there, with a mass six or
more times greater than that of the visible stars and galaxies, only we
have no way of detecting it, but really, it’s there? OK then.” Or
something like that, albeit in more formal scientific language.
These days, dark matter is a firmly established principle of
cosmology; most of the questions now focus on how the stuff is
distributed through the universe, and which of many possible subatomic
particles it’s made of.
Most of the questions, but not all. Ever since the early
80’s, a competing theory has been struggling for acceptance.
Known as
MOND, for Modified Newtonian Dynamics, it posits that dark matter’s main
effect — allowing galaxies to spin faster than they should — isn’t
caused by extra stuff, but instead by a change in how gravity works
under certain conditions.
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