Dark matter’s existence was predicted in the 1930s, when the astronomer Fritz Zwicky focused on a clump of galaxies called the Coma cluster. Based on the amount of light coming from the cluster, which is one way of estimating how much mass and gravitational force it had, one would have expected the galaxies on the edge of the cluster to move more slowly than those on the inside. But the galaxies on the edge were moving much faster.

Other astronomers later found the same phenomenon on a smaller scale. Stars on the edge of galaxies were orbiting much faster than would be expected based on the mass astronomers could see.

With dark matter’s existence now generally accepted, scientists have turned to the problem of finding out what it is. Black holes and stars and large planets too dim to be seen could account for some of it. But physicists say the answer probably lies with a subatomic particle or with particles we haven’t yet discovered.

“Dark matter is material that seems to interact by gravity but, as best as we can tell, does not emit any kind of light,” said Richard Mushotzky, a senior scientist at NASA Goddard.

The astronomer Edwin Hubble discovered decades ago that other galaxies seemed to be moving away from us in the Milky Way. The farther away they were, the faster they were going. But astrophysicists figured that gravity would cause the universe’s expansion to gradually slow down.

Then in 1998, two teams upended that theory by looking at Type Ia supernovae, rare stellar explosions that briefly release tremendous, consistent amounts of light. By carefully measuring how the light from these events shifted toward the red end of the visible light spectrum, they found that the expansion had accelerated in the last few billion years.

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