The other 27 percent of the universe is made of this stuff, which is easier to detect than dark energy because astronomers can measure its gravitational effect on distant stars. Still, astronomers aren’t sure exactly what dark matter is. Two competing – and cutely named – theories attempt to explain dark matter’s contents: MACHOs, for Massive Compact Halo Objects such as small stars, and WIMPs, for Weakly Interacting Massive Particles left over from the big bang.
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