Scientists don’t know what dark matter is, but they know it’s all over the universe. Everything humans observe in the heavens—galaxies, stars, planets and the rest—makes up only 4 percent of the universe, scientists say. The remaining 96 percent is composed of dark matter and its even more mysterious sibling, dark energy. Scientists recently found direct evidence that dark matter exists by studying a distant galaxy cluster and observing different types of motion in luminous versus dark matter. Still, no one knows what dark matter is made of.
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The experiment is the most sensitive in the world aiming to detect exotic particles called WIMPS (Weakly Interacting Massive Particles), which are one of scientists’ best guesses at what makes up dark matter. Other options include neutrinos, theorized particles called axions or even normal matter like black holes and brown dwarf stars that are just too faint to see.

WIMPS are thought to be neutral in charge and weigh more than 100 times the mass of a proton. At the moment these elementary particles exist only in theory and have never been observed.