One of the great challenges in cosmology is understanding the nature of the universe's so-called missing mass.
Astronomers have long known that galaxies are held together by gravity, a force that depends on the amount of mass a galaxy contains. Galaxies also spin, generating a force that tends to cause this mass to fly apart.
The galaxies astronomers can see are not being torn apart as they rotate, presumably because they are generating enough gravity to prevent this.
But that raises a conundrum. Astronomers can see how much visible mass there is in a galaxy and when they add it all up, there isn't anywhere enough for the required amount of gravity. So something else must be generating this force.
One idea is that gravity is stronger on the galactic scale and so naturally provides the extra force to glue galaxies together.
Another is that the galaxies must be filled with matter that astronomers can't see, the so-called dark matter. To make the numbers work, this stuff needs to account for some 80 per cent of the mass of galaxies so there ought to be a lot of it around. So where is it?
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