Where is the missing mass of the Universe?
The “dark matter” problem was first noticed by astronomers trying to “weigh” galaxies. They measured how fast stars orbit around a galaxy; knowing how fast it is moving and in how tight a circle, you can calculate how much force (gravitational force in this case) is needed to keep it there. To everyone’s surprise, the galaxies needed to be fifty times heavier than anyone had previously guessed. And this “mass discrepancy” started popping up in every astronomer’s observations. Huge galaxy clusters needed extra mass to explain the temperatures of the hot “x-ray gas” in their cores. Theoretical considerations of the age of the Universe didn’t make sense unless there was more mass than we could see. And the evidence suggested that the mass was not tied up in black holes and other normal, star-sized objects. Particle physics came to the rescue, sort of. Quite independently of the dark matter question, particle theorists had hypothesized about “Supersymmetry” to account for some oddities in
