Dark matter is thought to be an invisible substance that makes up almost a quarter of the mass in the universe. The concept was first proposed in 1933 to explain why the outer galaxies in galaxy clusters orbit faster than they should, based on the galaxies' visible mass.

(Related: "Dark-Matter Galaxy Detected: Hidden Dwarf Lurks Nearby?")

At the observed speeds, the outer galaxies should be flung out into space, since the clusters don't appear to have enough mass to keep the galaxies at their edges gravitationally bound.

So physicists proposed that the galaxies are surrounded by halos of invisible matter. This dark matter provides the extra mass, which in turn creates gravitational fields strong enough to hold the clusters together.

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